Sweet clam, and super sweet contraption. I keep mine in a tupperware in the bottom and that is pretty ugly compared to that.
IMO - it is unlikely that a snail killed your other clam. Usually, snails, shrimp, starfish, etc. will leave them alone until they start to die. If they sense that they are dying, then they will hustle over to finish them off.
Be sure and feed that guy 4-5 times a week with phyto.