If you keep finding them then they could be reproducing. They look like a nudi that would eat either polyps or leather corals. My guess would be polyps.
Guy is correct Aeolid nudibranch. They are predatory and are obviously reproducing in your tank. Scroll down in this article for more info on these guys. http://www.reefkeeping.com/issues/2005-05/rs/index.htm You'll have alot of searching/removing/dipping and inspecting to do to get rid of them all.
