If it's just one eye that's swollen, then as mentioned above it's probably due to injury. Fish get "popeye" in one eye because it swells due to trauma... "popeye" in both eyes usually denotes infection and requires antibiotic treatment.
What I would do in your case is dose some Epsom salt (1 tablespoon per 5 gals). Epsom salt is is just magnesium sulfate, making it reef safe, and should help the swelling go down.