Popeye is not just a cloudy eye - it is a condition and often a symptom of some sort of underlying disease...not necessarily a disease in itself. It can look like there is a bubble behind the eye, and results in a bulging eye. It really looks bizarre.
An animal that traumatizes its eye by scratching it or during transport or something can get an infection, a symptom of which is cloudiness or ultimately this "popeye" condition. But often there is an underlying cause. If it is in both eyes, there is usually some other infection or disease involved (and that is what kills it). If one eye, it is usually some sort of trauma to that eye. So in any case where you see popeye, you usually have to consider some other possible diseases.
And if it shows up in other fish, well, that is definitely cause for major concern. It could be a disease...or it could be another animal. For example, I worked someplace with a lunar wrasse that immediately picked at the eyes of any yellow tang it encountered. Unfortunately, this fish just ate the eyes out immediately...but another fish may just do this as a sign of aggression and could cause trouble for any fish. It is important to figure out what is going on.
