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:
https://www.reef2reef.com/threads/s...sease-treatment-and-diagnosis-section.775547/
Here is an article on fish eye health that I wrote:
https://reefs.com/magazine/aquarium-fish-the-eyes-have-it-a-review-of-eye-health-in-aquarium-fishes/
Jay Hemdal