Mandarins are pretty much safe from ich and velvet and such. They have a heavy mucous on their bodies that keeps those parasites at bay. Are you sure that it is ANYTHING at all? The reason I ask is that mandarins are notorious for 'snicking' foods (very much like seahorses) and expelling sand and other junk out of their gills. This stuff sometimes sticks on that mucous coat and looks like trouble, when, in fact, it is nothing at all.

Finally, mandarins that are picked on (nipped) show similar spots. It is a mix of the healing process and that same mucous that makes a film looking spot on them. It kind of looks like Elmers Glue that hasn't dried.

As Angie said. Unless you see serious stress or other indications of trouble, I'd just ensure that the water quality is good, that the fish is eating and acting properly, and monitor the tank to ensure nobody is picking on him.

Dave