Yeah - I had this with an Mbuna recently - both eyes - no warning.
From the reading up I did on how to treat it, I found it can be caused by stress, parasites, gas pressure in the water, bacteria etc etc etc - the list just went on and on. Having decided there was nothing I could do about most of these, I put an anti-bacterial dose in rather than do nothing and risk loosing the fish.
The fish recovered quickly - but I'll never know what caused it or cured it.
All I can suggest is that you try what I did as it's more likely to help than harm.
Give the salt a miss if you have a lot of plants, and I don't think salt will help on this occassion anyway.
Good luck with your fish.