Orange Shoulder tang; disease, or natural change?

[michika]

The fish's head has been darkening progressively, and its not a startle reaction, like I would see in say my sailfin, or foxface. I'm really starting to think its changing since the coloration isn't going away. I figure if the tang grows faster then I expected then it can go to the larger tank that houses the rest of my non-nano worthy fish.

My tang lives in a rock. Thankfully its not like my regal tang who loves to cram itself into the smallest spot possible.