Big black spots developing on the fish

[Ninja120]

I have a powder brown tang that has a big spots developing on his forehead and side. Right around the black spot there is slight fade in color. Does anyone know what it is?

 

[Jerry W]

Any way to post a picture of what you're seeing? Could be a number of things; stress, water quality,.
Post more info about your tank too; size/inhabitants/all water parameters/ diet/how long you've had the tang.