Question
I have recently been running a u.v. clarifier in my garden pond.  The water is still green although not Neon green.  My question is, what is the powdery green stuff that seems to settle on the rocks and plants?

Answer
Kindly follow the below test
Water Balance / Saturation Index Factor
Temperature + Total Alkalinity + pH + Calcium Hardness + Stabilizer + TDS =
Simply put, proper Water Balance means keeping ALL chemicals in the pool in the right amounts.  The following 6 factors affect waterbalance: Temperature, Total Alkalinity, PH, Calcium Hardness, Stabilizer and Saturation Index.  It is possible to have Temperature, Total Alkalinity, pH, Calcium Hardness and Stabilizer in the proper (ideal) range and still have unbalanced water because the Saturation Index may be too high or low.
     Goals of Water Balance:
?  Protect Swimmers from Irritating or Unsafe Water
?  Protect Equipment from Corrosion or Scaling - Pool water heaters in particular age prematurely - heat exchangers fail.
The Saturation Index (also referred to as the Langelier Index) is a major health indicator of your pool because it determines whether the water is damaging the pool surface  andequipment.
Water that is UNDER-saturated (Low saturation index) is aggressive,hungry and corrosive. In this state the water may cause etching, pitting, dissolving and staining of walls, grouting and plumbing.
Water that is Saturated (Zero saturation index) is in equilibrium, balanced and neutral to pool surfaces and equipment. In this state the water has no effect on the pool or equipment.
Water that is OVER-saturated (High saturation index) is scale forming. In this state the water will deposit excess minerals on the pool and equipment in the form of scale.
Since Calcium is the most abundant mineral in pools, it is the most likely mineral to be dissolved by corrosive Under-saturated water.Calcium is also the most likely mineral to be deposited on pool quipment in the form of scale from Over-saturated water.
Water that is only slightly scale forming is recommended. Ideally a very thin residue of calcium should form on pool surfaces.
The Perfect range for the Saturation Index is 0 to + 0.5 (slightly scaling).
Affect of individual factors on Saturation Index:
?  As Temperature Increases SI Increases
?  As Total Alkalinity Increases SI Increases
?  As pH Increases, SI Increases
?  As Calcium Hardness Increases, SI Increases
?  As Stabilizer Increases, SI Decreases
?  As Total Dissolved Solids Increase, SI Decreases
LANGELIER or SATURATION INDEX
The Langelier or Saturation Index is useful in determining the corrosive or scale-forming tendencies of pool or spa water.  In order to calculate this Index, it is necessary to have the following information:
    pH         Obtained by testing the pool or spa water
    TF        Temperature Factor - using the chart below, look up the actual pool or spa water temperature at the time of sampling, in order to obtain the appropriate TF value.
   CF        Calcium Hardness Factor - determine the PPM of calcium hardness (CH) in the water sample.  Using the chart below, use the appropriate calcium hardness (CH) reading, in order to obtain the appropriate CF value.
   AF         Total Alkalinity Factor - determine the PPM of total alkalinity (TA) in the water sample.  Using the chart below, use the appropriate total alkalinity (TA) reading, in order to obtain the appropriate AF value.
The ideal range for the Langelier or Saturation Index is -0.5 to +0.5.  Test values more negative than -0.5 (i.e., -0.6) are considered corrosive and steps should be taken to adjust the pH, total alkalinity or calcium hardness, in order to avoid the effects of corrosion.  Test values higher than +0.5 (i.e., +0.6) are indicative of scale-forming tendencies and steps should be taken to adjust the pH, total alkalinity or calcium hardness, in order to avoid scale formation and cloudiness.
The following formula is used to calculate the Langelier/Saturation Index:
Langelier Index  =   pH   +   TF   +  CF   +  AF   -  12.1*
       * -12.1 is a constant used in the calculation of the Langelier/Saturation Index
LANGELIER INDEX CONVERSION CHART
TF   CF   AF
Temp癋 / 癈       TF           CH PPM       CF           TA PPM       AF
32癋 / 0癈       0.0           5       0.3           5       0.7
37癋 / 3癈       0.1           25       1.0           25       1.4
46癋 / 8癈       0.2           50       1.3           50       1.7
53癋 / 12癈       0.3           75       1.5           75       1.9
60癋 / 16癈       0.4           100       1.6           100       2.0
66癋 / 19癈       0.5           150       1.8           150       2.2
76癋 / 24癈       0.6           200       1.9           200       2.3
84癋 / 29癈       0.7           300       2.1           300       2.5
94癋 / 34癈       0.8           400       2.2           400       2.6
105癋 / 41癈       0.9           800       2.5           800       2.9
128癋 / 53癈       1.0           1000       2.6           1000       3.0

The following example illustrates how the Langelier/Saturation Index is calculated:
   The actual pH = 7.4
   The actual water temperature = 78癋 or 25癈 : from the chart use TF = 0.6
   The actual calcium hardness (CH) = 198 PPM ; from the chart use CF = 1.9
   The actual total alkalinity (TA) = 97 PPM : from the chart use AF = 2.0
   -12.1 is a constant used in the calculation of the Langelier/Saturation Index
   Langelier Index  =  pH   +  TF   +   CF   +  AF   - 12.1*
   Langelier Index  =  7.4  +  0.6  +  1.9  +  2.0  - 12.1*  =  -0.2 (from above data)
   Results:  -0.2 is in the ideal range of -0.5 to +0.5
LANGELIER INDEX CORRECTIONS
The Langelier Index is a tool that is most frequently used by knowledgeable pool and spa professionals.  For the typical pool or spa owner, that maintains the water balance, within or close to the recommended ranges for pH, total alkalinity and calcium hardness, the Langelier Index should not be a concern, in as much as it will fall within the ideal range of  -0.5 to + 0.5.   In those instances where the pool or spa water chemistry is substantially out or balance or problems are in evidence, the Langelier Index can give insight into the proper course of corrective treatment.
Low Langelier Indexes can be raised, to within -0.5 to +0.5, by adjusting the pH to 7.2-7.6, the total alkalinity to 80-120 PPM and the calcium hardness to 150-200 PPM for a masonry pool or spa or 80-200 PPM for a vinyl or fiberglass unit.  A low Langelier Index can result in corrosion, bather irritation and discomfort.
High Langelier Indexes can be lowered, to within -0.5 to +0.5, by adjusting the pH to 7.2-7.6, the total alkalinity to 80-120 PPM and the calcium hardness to 150-200 PPM.  While it is always possible to lower the pH, it is not as simple with the total alkalinity or calcium hardness.  Lowering the total alkalinity usually lowers the pH as well.  Lowering the calcium hardness is not always possible, given the fact that some pools are filled with hard water.  In those situations, where the calcium level is high, attention should be paid to lowering the pH and/or total alkalinity as a means of improving the Langelier Index.  A high Langelier Index can lead to scale formation, cloudy water, filtration problems, heater problems, loss of chlorine efficiency and bather discomfort.
It is not always possible to get the pH and total alkalinity in range, due to the nature of the dissolved minerals.  pH is always more important!!!
To more correctly determine the total alkalinity, deduct 30% of the cyanuric acid reading from the total alkalinity test result