Without a picture and/or a biopsy I would take an initial guess of a fungal infection. However, it could be a bacterial infections which are also common in Koi, goldfish, and Koi/goldfish hybrids and marine fish. So I would need a real good description to distinguish between the two.
If you agree that it could be either of the above. Separate the fish into a hospital tank and treat it with a fungicide like Furanase, MarOxy, Ich-Attack, Rid-Ich+, Malachite green, or Lifeguard. Or treat it with an anti-biotic like Ultra Cure BX, BiFuran, Furanase, Trisulfa Antibiotic Powder, Melafix, Maracyn, MarOxy, or Lifeguard. (There are quite a few more antibiotics that I have not listed.)