Question
this is now my 4th year growing annuals for the summer under lights, and every
year i get a slime mold that severely dents my Impatiens population and affects
many of my annuals when they first sprout. i always let the top part of the soil
dry between waterings. is there a natural way to stop the mold?

Answer
Botrytis is usually the pathogenic nemesis for growers of Summer seedlings.  Slime mold attacking seedlings I confess is a new problem I have not dealt with in an indoor controlled setting.  Nevertheless, a few thoughts that may help you here.

Back in 1995, a California golfcourse began finding sick Grass all over its putting greens.  Experts at University of Arizona studied it, and after analyzing the DNA of the responsible mold, they i.d.'d it as Labyrinthula terrestris.  It was the only Labyrinthula known to attack on land.  All other Labyrinthulas live in salt water.  This new disease was named Rapid Blight Disease.  Scientists note that Soil Sodium promotes this problem.

Amoebozoa ooze along and burst into action when they reach a colony of favorite Bacteria.

That's but a few possible culprits in the Protists Kingdom. Please examine this gallery of Slime Molds to confirm this is in fact the kind of criminal you mean:

http://www.plant.uga.edu/mycology-herbarium/myxogal.htm

Bottom line:  I need to know a little more about what you have -- so I can tell you what makes it tick, and what you can do to avoid this problem in the future.  Either please tell me why you think this is a Slime Mold, or i.d. it (harder to do) and tell me what you think.  rsvp,