I haven't seen one of these guys in years. They look like a nudibranch, but I think it is actually a harmless cowry. (Correct me if I'm wrong.) They are a beautiful black velvet color. The strange thing is I haven't noticed this in the tank before and it is about 2" x 1.5" or it can look round. I think I remember seeing a picture of one in The Modern Coral Reef Aquarium Vol. I, but don't have a copy handy. Any information would be appreciated.