There is lots of good info on WetWebMedia
http://www.wetwebmedia.com/PopeyeCause.htm...
If I may loosely quote some info on that site... a true case of popeye usually affects both eyes and to treat would require an antibiotic....
Try QT'ing the affected fish, keep it in good water conditions and monitor till you discover what exactally it is... since fish dont have eyelids it is possible that the fish may have injuried himself by bumping his eye on a rock etc and causing trauma...
That is a small portion of what i have gleaned off that website researching some of my own fish-eyeball problems....
HTH