My clam has parasites..help!!!!?

I have had a fresh water clam for almost two years now. It has been doing fine untill a couple months ago I noticed its shell looked like it was being eaten by something. At first I thought it was my pleco but then today after its backside of its shell is completely eaten except for a thin layer wich is green, I took it out and realized a tiny tiny little worm/parasite on it. What is it? How do I get rid of it and make the little clam all better? I rinsed it off under cold water but i doubt that will work. I have no clue what to do but I need to save him from being eaten.. Ill be sad :(....Thanx