Does sound like a spawn. Rare but not unheard of. Never heard of any baby clams come out of one, alas. Change carbon, clean skimmer and a water change ASAP. Had it happen a couple times before w/p incident but last one I lost 4 fish. :(

Clams are known to spawn when they become very stressed out or are about to die (of course they sometimes do it just for fun like the rest of us too). One clam spawning will then trigger others to follow. Aquaculture facilities will deliberately do this to breed new stock for growout and sale. I've never heard of a case where someone was able to successfully raise baby clams after spawning in a small closed system home aquarium either. Like the others have already said, do a big water change as those gametes will break down quickly and really pollute your tank.

Giant Clams in the Sea and the Aquarium by James Fatherree is a good book on the subject.