Coral Eating Critters?

We have a 75 gallon saltwater tanked stocked with a variety of corals and fish and live rock. Its just under a year old and we have never had a problem keeping things alive, until recently. A few pieces of coral have been looking sickly, and after looking closer, I found some type of critter eating away at the coral. He was white, with small little flower shaped tenacles all over the top of him, and a suction botton (like a slug). We pulled him off the coral, and flushed him. The piece he was eating at did eventually die and disintegrated. Today, I noticed again a piece of coral looked like it was being eaten, so we took the piece out, and on the botton of the rock it was attached to was 4 of those critters! We have never noticed these guys before (just different types of worms in the rocks) and now they seem to be infesting our tank, and destroying it! We found a total of 6 and removed the from the tank. Anyone have any idea what thest critters are and how to get rid of them?