I think the four motoro stingrays we rescued had/have the same, but it's disappearing. I'm not sure what it is exactly, either. If it's bacterial, fungal, or something else. Is he eating alright? Does he have tank mates? What have you done so far? Pictures will definitely help
Feed him as much as he will eat.
Salt -- 3lbs. per 100 gallons.
Make sure water is pristine.
Eliminate any stress factors (tank mates, light, etc.).
Keep the tank dark.
well before posting this i hope you did a massive water change atleast 50% followed by another 50% water change.... this should allready be done... now it seems to me your A describing the slime coat pealing away or B disk rot PIX are needed but increase your temp to 86 ish and add salt 33lbs per 100 gallons as high as 4 lbs but post some pictures