I wouldn’t worry about the ATS at the moment as it’s probably not becoming very established anyways due to the lack of phosphates. It would probably be more beneficial to simply turn off the ATS light to allow phosphate levels to increase and the overall health of the tank to improve as a result. Once the cyano is toast and phosphates are elevated, the ATS light can be turned back on and will take off and the screen should be dark green within 48 hours. Then you can start feeding the tank like crazy. On a side note: Sometimes I get bored and scrub my screen until it’s nothing but the opaque color it came as. Within days it’ll have 1/8” of algae growth on the entire surface. I’ve had very little growth since I let my phosphate levels slide but the growth restarted immediately after I added a little Flourish Nitrogen. Phosphate is just one of those things where if you don’t have enough, it causes all kinds of problems. Just look at the coloration for example:
Good nutrient levels
(5ppm Nitrates, .04 Phosphates)
Oops
(5ppm Nitrates, Undetectable Phosphates)
(Note the biopellets that stripped out my Po4)
To think I'm excited about a fingernails worth of algae, you have a rain forest full.