My spotted sailfin gets that too, sometimes looks like white mold!! I trip out only to find he is fine as hour later. I've seen them at the lfs with this too, I think it has to do with the lighting. If it's in direct light for extended periods of time, it's black/brown fades on it's back. I notice in the mornings when the lights go on, he has a really healthy dark brown and black dots on him.
Just my opinion?