Looks like a slight bacterial infection. You could treat in a QT using a broad spectrum antibiotic such as Kanaplex or Furan-2. Or you could try soaking fish food in a vitamin supplement (exs. Selcon, Zoecon, Vita-chem) to help boost the fish's immune system to fight this off naturally. It looks to be a minor infection so I would try that first before resorting to antibiotics.