Clams are absolutley irrisistable to critters like crabs, shrimp and snails once they start to even remotely go downhill. Once the clam doesn't have enough energy to fend off predators (i.e., stay shut), it's feeding time! It is amazing how fast these guys go to work. The scent a dying clam gives off will attract pretty much everything in your reef to come and have a party.

FWIW, you can dip clams in FW with no ill effects as you would dip new corals in TMPCC or the like. It will kill off pyrimad snails and other clam eating invertabrates yet leave the clam unscathed. In your situation I'm not sure what caused your clams demise, but I can assure you it was your efficient C.U.C. that took care of it in the end.