Could be the possible beginnings of two diseases (see below). Or it might just be a cut that has gotten infected. Unfortunately, you are going to have to wait to see how it develops before knowing for sure. Is the fish eating?
Brooklynella:
Symptoms – This is most often seen in clownfish, but it can afflict any fish. The fish’s skin will appear to be peeling or sloughing off, oftentimes causing excessive white mucous to form around the affected area(s).
Treatment options - Formalin bath, followed by additional formalin baths (as needed - but give the fish a day to recuperate in-between baths). You can use formalin in a QT (at a much lower concentration than the bath), but great care must be taken to provide plenty of gas exchange as formalin will quickly deplete the water of oxygen. For this reason, doing baths is the safer option as the fish can be pulled from the formalin if showing signs of distress. Formalin-MS is preferred, as that contains 37% formaldehyde. However, in a bind, any medication containing formalin (ex. Quick Cure) is better than nothing. Alternative treatments for brook include metronidazole (ex. Seachem MetroPlex) and acriflavine (ex. Acriflavine-MS). A freshwater dip may provide temporary relief if you are unable to locate any of the aforementioned medications right away.
Uronema marinum:
Symptoms - These are the red sores often seen on chromis damsels. The disease seems mostly confined to damsels and clownfish, but I have seen some exceptions to that.
Treatment options - This is a very difficult disease to treat. Possible treatment options include: Metronidazole (ex. Seachem MetroPlex), acriflavine (ex. Acriflavine-MS), Chloroquine phosphate and copper. The problem is the fish can never be returned to the infected tank from which it came. Uronema is a “free living” parasite which does not require a fish host. So, going fallow will not eradicate it. Most fish seem protected from it via their natural immune system; but for some reason, chromis and some other fish are not always afforded this protection. Once a tank has Uronema, it must be assumed that the disease can survive in there almost indefinitely.
Formalin bath or freshwater dip may provide temporary relief for Uronema.