how did u attach them to the rock??? they may very well have slimed their way off of the rock if u attached them with superglue gel......most leathers do this(or at least in my experience) a good way to prevent it is stick a plastic toothpick through the base of the coral then put one end of a rubber band around one end of the toothpick and strap it around a rock over to the other side of the toothpick.......when i begins to grow and attach itself u can just cut it all off and take the toothpick out!!! voila!

hope it helps, steve

ps. what sort of inverts do u have in there, its not always fish that feed off of corals.