Looks like some kind of traumatic injury. Possibly something fell on him or another fish, pistol shrimp attacked him. These kinds of injuries will heal on their own in a healthy specimen with a good diet and excellent water parameters. if you have a QT, you may want to consider an antibiotic to keep the wound from getting secondarily infected. Also Mandarins do not have scales, so treating them may be different than other types of fishes.
Take a look at this thread for some ideas on treating with an antibiotic.
Stocking the Marine Fish Medicine Cabinet
All the beat!