and if it is snails how do i get rid of them?
At night, when the snails are most active...remove the clam from the tank, manually remove the snails with tweezers, and put the clam back. Do this nightly until you don't see anymore, then do it weekly and eventually bi-weekly. Without having inspected and removed any of the snails before you put it in the tank, there is no way you'll ever be sure you got them all, so regular inspection is essential. You should also get a sixline wrasse, a clams best friend (they eat the nasty clam munching snails).