I dunno reefnutz, Ive heard of stories (mainly fireworms) ganging up on fish and devouring them like a pack of wolves..Actually it was an article I read at my local LFS...Ill try to find it to quote it....
My aquarium is full of those worms for years. I cannot say for snails as I don't have snails, but I never had any problem with them and fishes/corals/clams/ursin.
If you look closely you can see that the head of the Fireworm is actually pointing out of the snail shell. It look to me like the worm was just looking for a place to hide.