Probably a bacterial infection. Hard to tell. Is it hard skin or soft tissue swelling? I’d start with some antibiotics just to see if it decreases in size.
Beautiful fish. White-ish filmy areas like this should be watched with great concern. Bacterial infections so close to the eyes, brain, gills, etc. can go sideways really fast. Consider treating this awesome fish with Sulfaplex + Neoplex for infections on the head area.
Wasnt there a thread about a clownfish just recently with very similar symptoms?? I also believe that things went sideways quickly. I think @Humblefish had some recommendations on the subject. I also think @Big G has some very sound advice..