I can't guarantee you they are harmful to small clams, but I personally would take any of the large Nassarius species out of the clam tank for at least a month to give the clam a chance to settle in. They still might kill the clam later if they get hungry, although there are a lot of people on here that will excuse them by saying the clam must have been dying. This may or may not be true, so use your own best judgment.
Cheers,
Don