This black stuff, does it look like a second skin over the coral?
Mine do this and it is shedding. It first looks like a protective covering all over, then as water movement from my Power Heads gets near the coral, this skin rolls into a long stringy type black colored substance. If you grab it with your fingers, it's slimy.
Anyways, don't use your hand/fingers to remove it with. Use a turkey baster and blast some of your water across it! Don't touch your corals if you don't have to.
29gal BB | 42+ lbs LR
| 143watts PC
| Asst. Leathers, Frogspawn, Hammer, XLG Open Brain, Zoanthids, Mushrooms, Diploastrea heliopora | Blue Mandarin | Atrosalarias Fuscus | Pair Captive Bred Ocellaris Clowns | Peppermint Shrimp | 6 Astrea's |