I am new to fish and bought serveral books. Pop-eye was a disease I read about.
In "The Everything Tropical Fish Book," the description is just that the eyes appear to be bulging from their sockets. The book says to check the water quality and not to feed for a few days.
In "Mollies, Keeping and Breeding Them in Captivity," the author writes that pop-eye was one of the diseases he first encountered before realizing that mollies need salt. They can die. It can spread, so consider separating the patient. If water quality is good, then it may be bacterial. The fish can recover but the eye could fall off. His lists remedies as fixing water quality and treating with antiboditics.
I have never seen this but I did just lose my mollies to dropsy. I used the mollie book for guidance as to ending their suffering. The author suggests putting the fish on wet paper towels and putting into a zip lock bag and into the freezer for a painless, permanent sleep. This method also provides a "coffin" for when children are involved.
I wish you the best, and I hope your story ends better than mine.
Hmmm....looking at your sig. if the fish is the mollie in the 5g then I'd say quite likly a water quality problem as it looks like this tank is way overstocked