It sounds like your goldfish has the symptoms of dropsy. It 's a condition where the body usually bloats up and the scales protrude. You are doing the correct thing by separating it and treating it with aquarium salt to help it remove excess water from its body. One of the causes of dropsy is a bacterial infection so you can try to treat it with antibacterial medicated food to see if it helps.

the slime is probably a by-product of your fish's infestation of the bacteria causing the dropsy. it could also be a product of another disease; pretty much anything from flesh-eating bacteria to mild ich. i have no picture, therefore, no idea.
check his eyes-look for veins and see if those are bloated like his body. if they are, then you need to really watch him for anything unusual...if they(the eyes) are non-blacked, look for ANY sign of red near them.if you see that, my best advice is to go into a dark corner for a while and be sad before going back to tend your fish.