It's not a good thing could be ammonia burn or and injury from a bite or hit it on something. Very easy to fix put it in a hospitable tank and add 1 teaspoon for every 5 gallons of water of salt then add some melafix and it will clear up in about 3-4 days. Up the temp to about 82.
Is it on only one eye or both eyes? If your water parameters are fine and only one eye is completely cloudy, then I would suspect infection due to eye trauma (fighting or scraping on some object). Make sure the water is kept as clean as possible and add salt, it should heal in a week or two depending on the severity.
If both eyes are cloudy and it's not ammonia, then I would suspect other water parameters... Has there been a sharp drop in pH?