Sounds like something has given off a toxin, possibly from dying, being cut, or alleopothy etc. Toxin release was most likely cause of clams death. Run carbon and change the water.
I read that clams are really sensitive to toxins given off by zoa's, palythoa's, and leathers. Cutting these corals (fragging) in the tank can release these as well as die off of same.
I lost a clam last year when a finger leather started to die and give off toxins. That is one of the reasons I run carbon 24/7. Hope that helps.