Sounds like hydra to me. Does it look like sort of a little white "tree", where it attaches to the glass (or plants or driftwood) in one spot and then has little tiny "arms" almost? If so, that's hydra.
If that sounds like what it is, then check out this info: Hydra Elimination in Shrimp Tanks
If it's something else, then...hmmm...a picture would be really helpful.