I noticed today that my Gem has a little faded color on the top of its head. He does not fight anyone in the tank l and doesn't seem to be stressed out. He eats like crazy and is one of the dominant one in the tank.
I wanna say bacterial but known as a thumbprint can be stress, poor diet, water quality (such as high ammonia or nitrate) and post injury
My best guess is water and diet
What foods are you feeding and what test kits are you using?