If just the one eye is affected, this is most likely from a mechanical injury - the fish gets frightened, hits a tank wall or something and the eye swells up. Assuming no other issues, and the fish still eats o.k., this can get better in a few weeks. Sometimes, secondary bacterial infections start in, and then you should treat with erythromycin (Maracyn 1) in a treatment tank.
The just is still out as to if it is better to hold affected fish like this in a acclimation tank, or put them back into the main tank.
This fish may have multiple issues (I ca see some white marking on its sides) In order to delve into that deeper, I need some background information, as outlined in this link:
Aquarium Fish Disease Problem Solving Determining the best course of action for a problem with your aquarium can be a difficult task. Even advanced aquarists will often need to seek the help of others for complicated or unique issues. Our intent here on Reef 2 Reef is to offer expert advice...
www.reef2reef.com
Here is an article on fish eye health that I wrote:
Aquarists need to inspect their fish closely every day, for signs of impending health problems. It is much easier to resolve a problem when it first starts, than to attempt a drastic emergency treatment when the fish is close to dying.
reefs.com
Jay Hemdal