As Jay pointed to, it is likely an injury. Keep optimum water quality and feed a high quality diet and it will likely heal on it's on. I'm not sure what you are feeding and your tank set up, offering live fish feedings of guppies or mollies is the best for the immune system, as the concern would be for a bacterial infection. Some suggestions for dead food, salmon and Hikari silversides, stuff the silversides with a pelleted food high in vitamin c, vit c helps in healing. Hikari also makes a marine carnivore pellet that is high in vit c, but use your own favorite. Salmon is one of the most nutritionally dense dead food for the health of lions.