ok. I couldn't find any old photos of it so I couldn't be sure how new it is or not, but I first noticed this about a month ago. My best guess is something due to nutrition since I have been having to travel a lot recently leaving my fish with an automated feeder and flakes (mix of Cyclop Eeze, Formula One, and Prime Reef). I am planning to get some Selcon to put on its frozen food now. And there is some nori in the tank for it now too. So hopefully the better nutrition will fix whatever this is.