Clam Question

[Marlin65]

I have 3 small clams and I had 3 big ones.
One of my big ones died it had a large hole in the foot like it was eaten.
Now I am having the same problem with another of my big ones (About 3")
The small ones are in a different tank and they seem to be OK.
The larger ones were in a different tank on the same same system. I have taken them out now and moved them to another tank.
I have been having some problem with those pyrite snails, and have been trying to eradicate them. They show up on them about ones every two week and I have to clean them off the shell about 4-5 at a time.
Neither ones with the hole in the foot had any snails on them though.
So I am wondering if this was caused by snails or if I have a different problem. There seem to be some white pinheads on them that I am not sure what that is. Not sure if that might be a problem.
My parameters are all good so thats not a problem.