Its really hard to tell but it could be a fungal infection. If this is the case, all fish in the tank need to be treated as well.
To do this you would want to do a full water change and treating the water with a fungus eliminator( I'm sure you could find one at the petstore). You should do this process every three days until the symptoms are gone, than use bettafix or something stronger to zap the water the kill any remnants.
If this is ich ( which looks like little white spots all over the body) than all fish in the tank should be treated with ich medication.
Like I said in the beginning it it hard to tell but these are two possibilities, I hope it helped !