It sounds like fungus. I've never had to treat it, but you can set up a qt in a hurry in any spare glass or food-grade plastic vessel that you can get a bubbler into. You just take water from your regular tank and discard it all when you're through. A spare heater is good; lights aren't that necessary. There are net 'dip' treatments, in which you use a dedicated new net to dip the fish into a bath, then qt. particularly until the dip has washed off and the fungus is gone. I'd wait for another answer on this, as this is all hearsay, but since no one else had answered, I thought I'd at least provide some suggestion. Mandarins are fussy eaters, as you probably know, so you might have to find some copepods for him while he's in isolation, and the copepods probably wouldn't survive the medication if you have to put medication directly into the qt tank---that's one reason a net dip medication might be better, because the little that gets into qt is minuscule by comparison. The last time I had to deal with this was 20 years ago, so I'm sure both medications and techniques have improved: hence wait for someone who knows better than I do. I wish you all luck with him.
