I agree that some pics under white light may help with an ID here. The pic looks like it may be Coral-boring Spionid Worms, but I’ve only seen them in soft corals once before:
From my many encounters with it, peroxide seems to help but it will go away on it's own. Somebody put the white hairs under a microscope and it was alive. No clue what exactly it is but it always bothers zoas.