My Blue Tang acts and eats normally, but looks like this. Unsure why or what to do. He’s in a 55 gal. with a Gold Head Gobie, six line wrasse and a pajama cardinal. Also a number of snails, a sand sifting starfish, a pencil urchin, and a coral banded shrimp live in the tank. He was fully QT with all the fish about six months ago (kanaplex then Prazipro then copper power and finally a fresh water dip) after something ran through the tank that killed most of the other stock. Was in the QT together for the full DT fallow time before putting them back. No other fish show this appearance or any signs of problems or distress. Water parameters all within normal range except my nitrate stays higher (45) but potassium within range. I do a 6 gal water change every two weeks and have Marineland 360 filter which I service about every 90 days. Also have a HOB protein skimmer. Any ideas or suggestion will be appreciated.