Hi, welcome to Reef2Reef!
How long have you had the mandarin? It is very skinny, probably beyond the point of being able to reverse it. Once they reach a certain point, they utilize lipids in the liver for energy and that organ is damaged, and then there is nothing you can do for them.
Are there other fish in the tank? I don't see any small white spots that would indicate ich, but the pictures aren't real clear, and if it is ich, your other fish would be at risk.
Jay