Hi there. The poor guy. I'm glad you were able to take him so he doesn't have to suffer there on a lousy Walmart shelf.
Looks like shedding slime coat and probably columnaris which mimics a fungus because of all that fuzzy stuff. The only true fungus we regularly encounter in our aquarium fish is saprolegnia and he definitely doesn't have that.
Most likely his real problem is internal. :( The med you want to get right away is API Furan-2 to treat the external columnaris (it's at Petsmart). If he hangs in there, the other med you'll need is Seachem Kanaplex which unfortunately usually has to be ordered online (try www.drsfostersmith.com
). Kanaplex is absorbed through the gills and skin and will treat the internal infection, which Furan-2 won't. I'd also put him into 2-3 tsps of epsom salt per gallon to help keep fluids from building up in his system.
I will be honest because I don't want you to get your hopes up: his chances of survival are pretty low because of the way he's just laying there. :( But you are an absolute angel to try to save him and at the very least, he will get to spend the remainder of his life in clean, warm water with someone who cares about him.
Good luck with the little guy and keep me updated.