One cloudy eye is probably due to an injury. You can feed vitamins to the fish to help boost their immune system while it heals on it's own. Keep an eye out for severe infection and it spreading to the other eye. If it starts to swell, you can add Epsom salt to the display to help bring that down.
Epsom Salt: Treats eye and other injuries. Popeye in one eye usually means injury, but popeye in both eyes usually denotes a bacterial infection requiring antibiotic treatment. Cloudy eye can be a bacterial infection or flukes.
How To Treat - 1 tablespoon per 5 gallons of aquarium water. Repeat only once, if necessary. Use only “USP” grade Epsom salt.
Pros - Reef safe, as Epsom salt is just magnesium sulfate.
Cons/Side Effects - The effects of excessive sulfate in a reef aquarium are not fully known.