my tank has been established for about 9 months now. everything is growing great. about a month ago i started seeing what i think is some type of algae growing on my onion plants leaves. it looks gray in color and like little tiny tree branches. it is very tough and well attached to the leaves. i can pull them off the leaves without causing any damage. however, they seem to be spreading rapidly now. mostly on the onion plant and very little on any other plants. does this sound like anything anyone has encountered and if so, is there a fix other than manual removal.