May just be because your screen isn't working right. Some exudates from algae can propagate other algae growth (as well as there own). There is lots of documented evidence that algae exudates can increase or reduce other types of algae growth. Have a look at my Allelopathy thread. The "Terms of engagement" will change if you can get green to grow. The Hydromentia horizontal systems report N and P in the 30parts per billion range, and algae just about persists, so the levels attainable in aquariums may be a bit of a red herring, so to speak.