It's weird and I have been wondering this for years. I use to think it was maybe nudis or parasites so I would always try dipping/medicating (Revive, CoralRX, Iodine, Vitamin C, etc.) to see if that helps, but there is never a clear resolution.
At this point I have come to notice this only happens to my PE type polyps, RPE, GPE, RDPE, PPE, Magicians, ASM, etc. etc. Usually they just end up 'correcting' themselves, some do it in a couple hours, some a couple weeks, but I haven't noticed any deaths. I think it might be this specific species.... Has anyone noticed it on any other types of polyps/zoanthids? If so, which?