fish disease?

Hi,

I have a yellow tailed blue damsel that appears to have lost all of
the blue coloring (scales?) at the top of his head and around his
eyes. So he's mostly blue, but black in that area. He's been that
way for about 3 weeks, but he eats well and is very active, and
otherwise normal. All water parameters are normal. Anyone have any
idea what this is? I'm aware it's just a $4 damsel, but I've had him
almost ten years and I'm upgrading from a FO tank to a bigger reef
setup, and I'd like to see him make the switch. Any help would be
appreciated.

Thanks,

Grant