This is popeye and is usually caused by a few different things. The fish may have injured the eye during an aggressive encounter with another fish or simply by bumping into a rock or coral decoration. The other reason may be a bacterial infection going on.
How is the fish looking otherwise? Eating and swiming normally? Is there appearance of any other type of disease process going on besides in the eye area, such as red streaks or bloctchs around tail or fin areas? Are the gills normal and breathing ok?
You can treat the fish for popeye with Maracyn2, but unless you have a strictly FO tank, which means no LR or LS, you will need to QT that fish in a separate tank. If you have a QT, even if your tank is FO, better to separate the fish and treat, rather than medicating your tank.