Question
How do I clean the green algae which is growing around the huge quantities of pebbles in the fish pond?

Answer
You can do 1 of 2 solutions:

1)  Physically remove all of the rocks and scrub them all by hand.  In this same sense, you could drain your pond and use a pressure-washer to clean them.

2)  You need to do a whole lot research on the food sources of algae.  Then, test your pond water for all of the nutrients that algae could be using to grow.  Then, figure out a customized plan that will safely rid your pond water of excess nutrients.

Both ways could be a huge pain.  The easiest, but not always most quickly effective, way is to add shade-providing plants to your pond.  They not only block out the sun, they also use nutrients that algae would otherwise use to grow.