Hi Julia, when is the last time he shed ? Did it appear all of a sudden ? It does look like Y.F to me, I really hope it's not but it looks like it. To be on the safe side, get some betadine from the drug store and mix some with warm water until it looks the color of iced tea to soak him in, about 1.5" deep. Don't let him drink it....offer water with a dropper on his snout first to be sure he's not
thirsty. You should also look up an experienced reptile vet to have a look and maybe do a skin scraping.