You need to get your chlorine up to 20 ppm and keep it there until you're no longer losing chlorine from sundown until sunrise the next morning. This will mean testing 2-3 times a day and adding whatever amount of chlorine you need to bring it back up to the 20. The key to killing the algae is sustained chlorine levels--just shocking once won't do it. You'll see the algae turn white/gray like JohnT mentioned, and your water will become somewhat cloudy. Keep the filter running 24/7 until it clears, and the green will stay gone (provided you keep your chlorine high enough afterward).

Janet