I am thinking it is a pest of some type there are several pests and dips to help with them. Coral rx is one of the most gentle and effective that I have read about. I have revive (which smells just like pine oil), iodine and flatworm exit. Depending on where I get my corals from will make a difference what I dip them in. I always flatworm exit dip my corals. If it is zoas I will use revive just because it helps knock off any adult nudibranchs, I don't have access to heartworm medication or I would dip all my sps
for red bugs before they go in the tank. One person I have gotten corals from actually dip them before bringing them to swaps, he has very high quality stuff and a great price. I do dip his anyway in iodine just to put my mind at ease.