A single cloudy eye is almost always due to an injury, except in some rare cases where a Neobendenia fluke attaches to one eye - like in a light infestation....kind of an oxymoron though, because infections soon turn critical.
If the fish has no other sympetoms (rapid breathing, frayed fins, cloudy skin) and *if* the cloudy eye showed up all at once, I would right it off as an injury and just wait and see how it heals.
jay