Is your fish improving? My thought would be some sort of bacterial / fungal growth. I've seen mine get something like a fungus / excess slime layer on the body which usually clears up by itself. If it persists (I've got one problematic one), I'll treat with a low level med like pimafix and that seems to clear it up. It can look really bad and then be gone in two days which is why I suspect either excess slime or fungus of the slime coat.
If yours is more a white colour of the body and looks to be penetrating into the body, I'd suspect bacterial infection.
A water test either way to check the tank stability just in case as well.