they can over-take your system too Ed. There have been some nightmare stories about this so what I do is whenever I am aquascaping or see a problem, I nip them off with a coral nipper. I had a really large one make camp in the middle of the trumpets I got from Colleen. They did not do any damage to the coral, but I know that they are at the minimum annoying and can turn into a real infestation.
I just pulled the coral head, and detroyed the snail. This one had grown to a very large size with teh majority of its calceferous tube deep inside the colony, so it took a while to deal with and it was about the size of an earthworm.
My advice: no need to panic, but I would exterminate with extreme prejudice.