I feel this is because
1) LED intensity will overpower growth initially for a certain period of time. Until you get a deep base formed, your growth will not bounce back from cleaning.
2) plastic canvas, while idea for waterfall screens, is not ideal for submerged UAS use. I'm going to go out on a limb here and say that it just takes time for the algae to root into the actual material on a UAS, then when it does, it can absorb the incoming light better and grow faster after cleaning. My screen has become progressively more 'stained' green, and this appears to not be so much staining as it is impregnation of the screen with algae spores. So after about a year roughly, it seems that the screen might hit a turnover point where growth can easily bounce back under high intensity LED light.
The difference with the waterfall screen is that the algae can form a mat in the holes and for some reason will hold on better in there, and the growth bounces back from that base and not so much the actual screen material itself. I have to scrape my waterfall screen hard. I barely have to scrape my UAS screen, even after 1.5 years of running it - most of it comes off just by grabbing it.