Bristle worms are great for the tank. They are astounding scavengers, and will make quick work of any dead animal...almost no trace after 24 hours. That said, they like to hide in and under rocks. I always found them hiding under my clams...and tridacna's have a wide opening..the size varies with the species...underneath. So the worm may have found a nice shelter, and possibly food if anything was stuck on the clam's slime. On the other hand, if your clam is dying, bristle worms will definitely be attracted, and as it dies they will clean it out thoroughly. If the shell is empty tomorrow, that was the case. You don't describe how the clam looks...wide open? fast reactions? Or is it slow, with the mantle retracted? Take care, Eric