I have a 29 gallon fresh water tank with live plants. The tank has recently developed this white cottony kind of algae looking stuff growing on the gravel and on some plants, mostly the java moss. It looks like heavy duty spider web. Some of it is almost 2 inches long swaying in the current. It is not from overfeeding, this is way different. Scooped out a small bunch and it seemed kind of slimy, almost mucousy, it does not have an odor and so far fish do not seem to be affected. I have had tropical fish since I was in fourth grade, 1966 and have never seen it before. Any ideas? I cannot get a clear enough picture to post.
Sounds like a fungus. Try partial water changes for a few days to see if that helps. Also make sure you remove any dead plant leaves or other debris from the tank.
Do you have a fermentation based carbon dioxide generator? If sugar or yeast found its way into the aquarium, it would grow that kind of stuff.