Its effective against green water (GW), but so are other methods. imho, correct nutrient, light and CO2 balance is the key to preventing algae blooms. However, you can't eliminate GW through nutrient or CO2 control. There are a few methods you can try, UV being one of them. Other methods are chemicals (ick), aka. clumping agents, and blackouts. A 2-3 day blackout should be effective enough to eliminate GW. 3-4 days should be able to elminate most forms of algae. With blackouts, what you do is clean your mechanical filters before the blackout. Raise spray bars, add air stones, etc. to increase surface aggitation. Do a 50% water change. Dose with KNO3 to get 5-10ppm. Turn off your CO2. Cover your tank with a heavy blanket, or black garbage bags. Wait 2-3 days, don't feed during this time, your fish will be fine. After 2-3 days, do another 50% water change and redose your KNO3, lower your spray bars, remove airstones and turn your CO2 back on again. UV is usually used for very stubborn GW, when blackouts don't work.
HTH
-Richer