I had to take about 60 pics to get these 2 that showed this accurately. They are full-res pics so you can see it. There appears to be a lesion of some kind that is about half the diameter of a penny, just under the skin, you can see the discoloration. Protruding from this is a white pustule of some sort. I can't tell if this is damage from running in to something, or if this is the dreaded "I-word".
So you can see in the pic without the flash, it looks white. This is how it looks under normal light.
The pic with the flash makes it look dark.
The only thing I have done different is that I put a canister filter on the tank and cleaned the glass really good, then pulled a dead coral colony, moved some rock, moved some corals, and left the canister running for 2 days.
I suppose he could have freaked out and bumped into something, or something could have just gotten 'caught' on his skin. He's eating fine and otherwise up front and personal.