Green water (algea blooms) will quite often sneak up on you. A lot of times the algea makes the water look a little cloudy, but not green and then one good light cycle and you have pea soup.
Next issue, once the bloom occurs, water changes will not make it go away. You change the water and dillute the alge level, then it grows back ASAP and your water is still green.
To kill the algea, a 4-5 day blackout is the easiest route to take. cover your tank with something opaque (posterboard, cardboard, blankets, trash bags etc.) make sure that all light is absolutely stopped from entering the tank. any light at all can perpetuate the problem.
Next is prevention for the future. If you are running a Fish only tank, back your lighting down as much as you can and still enjoy your fish. the fish are happy with ambient room light, so you really only need the lights on when you want to view the fish. If you have plants then re-think your fert schedule to better balance the nutrients in the tank
Algea thrives on excess nutrients, and imbalances in the tank. If you have no plants, everything is an excess nutrient. If you have plants you will want to re-think your Ferts, and post some questions on the plant forum. Either way, water changes are in order, and some testing. Do several high volume, water changes before during and after the blackout. Get the tank parrameters in very good shape Target nitrates at about 5 ppm for starters and vaccum everything you can out. It isn't necessary to feed your fish during the blackout, and it will help your efforts to skip feeding during the clean-up period.
You will also want to re-evaluate your feeding routine and amount to prevent future blooms. Any excess in the tank can contribute.
BTW, the one thing going for you with an algea bloom is the fact that it doesn't hurt your fish. So you don't have to take drastic measures to protect them from the algea.
dave