Good morning, I just started with my very first saltwater tank about 1 and half month ago. I just bought a yellow tang from a local store last Friday but I have been noticing that he is acting different. He started hiding behind the rocks and eventually was constantly in one of the tank’s upper corner close to the surface. I am new to this but I have noticed that his skin seems somewhat grayish and with some dark spots throughout the body and I don’t like it. Can anyone tell me what might be happening to him? Thank you in advance.