They do look like a blue or purple clove polyp. Yes they do quickly spread and are very hard to control once they have spread. They should be sold with a warning. Sorry I can't offer any suggestion on getting rid of them that would be a solution you would like to hear. The only sure fire way to get rid of them, that I know of, is to take the rock out to kill it, which in a large reef tank is not the preferred solution. I have read of success with certain angelfish, of course, they may eat some of your other stuff as well. I hope someone else on here can offer you a solution. Have you tried anything so far? They don't appear to be all that healthy compared to your surrounding corals.