I have a purple tang I’ve had him 4-5 years -he has grown well and been in great health. I’ve read about ick and velvet. This isn’t what it is. If it was a person I would say a bruise rather than measles or chicken pox -it’s a white spot/area-under his fin and a small area above one of his eyes. It has just in the last day or two shown up. It’s only on one side and he seems to be fine. He’s eating - breathing -swimming normally. I went through your common sickness and the closest is a bacterial infection. I have a few pictures-but can’t figure out how to attach them-as of now I don’t see this on any of my other fish