It is possibly some type of lesion indicating a pathogen of some type. It could be a secondary infection from a wound or a parasite of some type.
If the skin appears to be broken, it is prudent to add aquarium salt to the water at a rate of 3-5 grams per liter to aid the fish's osmotic balance with the water as well as a possible prophylactic treatment. Salt can be effective against fungus and several types of bacteria at this concentration.