I have a large blue damsel ( unsure of species ) that formed something similar, suddenly and for unknown reasons. We left it alone, fed well, good water, and it is gone now. Took time, months, but the eye is normal (almost) and is not blind. I think there is little you can do with eye injuries. If it is gas filled like a shiny bubble it could be treated with antibacterial meds as a possibility. I would do this in a qt. only. Gently handle the fish if attempting this, as damage to the effected eye usually worsens the situation.