Was it in the middle of the night? I notice my fishes go really pale when I peak at night when they sleeping

was afternoon, lights would have been at max for a couple of hours by then

That's a neat trick! In 6 years I don't think I have ever seen my purple tang do that! I've seen him get less-purple when he's freaked out (fighting with a new fish, etc), but not WHITE. Wonder if it's a pigment mutation.