right now if your other clam is still alive put it on a shelf up high , if its not in too bad a shape fresh water dip it. look inside the clam to see if snails are on the inside and watch it at night(like 3 am ) to see if snails are trying to get to it.

I have 4 clams and also have pyrimid snails. I've kept then in check buy manual removal
Like Denny said a few wont do much damage but left unchecked will multiply.
Once a week I pick clams up and loo at the foot and very edge of the shell I pick off snails and brush with toothbrush to remove eggs, of coarse in dif water.

Mystery, coris, melonarous wrasses are your best bet for long term.

Thanks guys, I took the clam out all together. It was dead :( I guess if anything I learned something new out of all this....don't keep clams!!

look into a six line wrasse for pyramidellid snails