Hi, it,s difficult to be sure from a photo but it looks as if it could be bacterial infection from a previous graze/scratch, I have had fish that showed symptoms similar to those in the photo that presented with internal bacterial infection within days so swift action is vital !! NT labs do a product called Anti Internal bacteria here in the UK that is almost a miracle cure for this ailment, if you can aquire some I highly recomend it-it is a permanent in my medical kit! If there is "fur" present on the wound then it is fungal, either primary or it could be secondary infection, I would use a dose of Pimafix even if fur isn't present as a preventative.
Hope this helps,Mike.