Ok, so...I did take the cloth off again. This time I think permanently. There is actually a significant amount of growth in the scrubber now which should act as some kind of obstruction so that it isn't so bright in there that it can't grow. Maybe that's what you really need to take the cloth off. Now whether that even means I'm gonna be scrubbing more, I don't know. It might scrub less again and I might have to put the cloth back on, yet again. But I think I'm gonna go without at least for a while, like two weeks. By then I should be able to tell if I've grown any more.
The algae in the tank is being slower to come back. I guess I should be happy about this but somehow I really don't think this is over! Also the rocks are still horribly ugly. I just don't want to mess with the tank now that it's headed in the right direction. Arguably scrubbing the rocks now could release tons of nutrients back into the tank which could conceivably fuel more growth in the display.
I think grabbing huge tufts of hair algae off the rocks is definitely worth it, because it removes so many nutrients per how much of the rocks and tank you're disturbing. However, my idea of taking like a steel grill brush to the rocks I think is wrong. For all I know, the rock is just hiding even more phosphate underneath a very thin depleted outer layer. So my current theory is that scrubbing rocks is wrong. Or at least not worth doing. Maybe you could get away with it if your nutrients are so low that releasing a bunch of phosphate isn't going to do anything anyways.
One theory as to why my rocks were growing so much algae is that they'd been absorbing phosphate this whole time as a result of my nutrient build-up. Probably once my little drop scrubber stopped scrubbing, the nutrients started getting taken up in both nuisance algae and the rock at the same time. Maybe that shows just how many nutrients can build up in a tank with 0 water changes. Literally so high that the algae can't take it all and rocks themselves start absorbing it. So what I've been pulling out (as algae) is all that phosphate buildup.
Many of these things are things that nobody told me about. Because nobody tries to run a 60 gallon system without water changes (that actually has fish in it). But what have I actually learned from all this? I think that I've learned that algae scrubbing has to be taken very seriously in this situation. If you're trying to pull off a 0 water change tank with next to no nuisance algae, those nutrients are going somewhere. They will be taken up by algae whether it be in your display or in your scrubber. Unless you want to go the complex and probably even more expensive route involving a maze of reactors, which have ongoing costs. But yeah, if you're trying to rely on one singular system to export your nutrients, you're going to need a very powerful scrubber. The moment the balance shifts in favor of the display tank, the display algae will out-compete the algae in the scrubber, rendering it next to useless.