What is this on this kole tank?

[Tangs Rule]

I have seen white spots before, but not dark patches like this. Does anyone have an idea what this is? The tang is still swimming around like a tang and eating everything I put in the tank. But these patches are ugly.

 

[Bob 1000]

Looks like stress... I've had tangs come in like this but it usually goes away, If the water Pars are kept good..