Heliofungia spp. Large tentacled plate corals (long - tentacled plate corals) Need bright illumination, do not do well under dim lighting. Zooxanthellate but must be fed. Probably prey on small fishes in nature and prefer fishes to crustacean food in captivity. large pieces of fish can be placed on the tentacles and will be hopebully rapidly eated. Easily kept if well fed with frequent feedings of whole fish. Live on sand or coral gravel and need same substrate in aquarium. not as mobile as Fungia, nor as aggressive. often do not recover from being stung, so place them where this is not likely to happen. do best in moderate currents. is it possible that the piece got stung or contacted by coral mucus slough off. sounds suspecious as you by far have some experiance with this. must have been contacted with something or damaged in some way. no advice on what to do as they are difficult as you know especially if injured. I would continue to feed it, watch to see if something is picking at it or worse, and review your lighting, water movement, and water parameters. wait it out as best as you can when you cant stand it any longer and the piece is not recovering remove at your discretion. hope something here helps. I know you are aware of most of this but sometimes it helps to hear it again. good luck. pics would help.
