Could be a "snowstorm" caused by dosing calcium mixtures way too much.
Are you dosing with calcium or a 2 part buffer?
This may be what's on your coraline, but the false perc I just don't know for sure.
Just a guess!
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 |