GFO is a filter media so to speak, granular ferric oxide. Used to lower phoshphate levels. Sounds like pyramid snails. A couple snails no problem, but in the home aquarium they rarely have any natural enemies. The population grows and the damage to clam causes too much stress. If you look at night a few hours after the lights go out, there are usually many more. Sorry about the clam. You are going to have to figure how to proceed, because your other clams most likely have them as well.