The cause for green algae is probably long gone, as Green Water generally resolves the original issue. Most of the time it's caused by a spike in Ammonia. Unfortunately once Green water is established, it can be very difficult to get rid of. There are 3-4 ways to deal with Green Water.
1. Reduce lighting to 8hrs per day, ensure CO2 levels are at least 30ppm, ensure that nutrient levels are within targets, and perform regular water changes. While this method does work, it's not for the impatient as it can take months to take care of the problem.
2. Perform a 3-5 day blackout. Perform a large water change right before the blackout. Make sure the aquarium is completely covered so that no light can get in, no peeking, and no feeding the fish for the length of the blackout. Perform another large water change after the blackout. Ensure that CO2 is at leat 30ppm and dose nutrients to target levels. If the blackout isn't long enough it may not completely knock out the Green Water and may need to be repeated.
3. Use a Diatom Filter. This will filter out the algae. May not completely eliminate the Green Water the first time, so multiple treatments may be necessary.
4. Use a UV Filter. Kills the Green Water that passes through it. Since the dead algae is not removed it may cause another Ammonia Spike causing another Green Water outbreak.