Sorry to hear about your hippo. It sounds like typical "popeye". It's possible that the fish got an eye injury, especially the way P. hepatus (your hippo tang) like to wedge themselves in rocks and crevices. A bacterial infection could be present also.
You might want to leave it alone, and work to reduce any stress which could include keeping the water quality pristine and providing enriched foods (like PE mysis soaked in Selcon or Zoe). If it doesn't start to become better in 2-3 weeks or if it gets worse it might be a good idea to move the fish to a hospital tank and treat with a gram positive antibiotic such as Erythromycin or, if that doesn't help, a broad spectrum antibiotic such as Doxycyline.
If you do try and catch the fish don't use a net or anything that could cause additional injury to the eye. A small plastic strainer is one of my preferred fish handling tools.
Good luck, keep us posted.