Cleaning your filter too much (every water change) kills all the good bacteria and you get a tiny ammonia spike which triggers the green water. If you really want to get rid of it you can use a diatom filter. This is the way I deal with it. Or if it is in my planted tank I just dose KNO3 and its gone in a few days. But diatoms tank it out of the tank. Since if you kill it and dont remove it the dead algae causes more ammonia and the cycle repeats. At least that is my experience.