The nutrients are just building up in the system, which is causing the algae growth.
I would suggest keeping all your powerheads on all the time to help with low flow and getting as much uneaten food and poop in the water column until it can be filtered out rather than giving it the chance to settle and decompose.
I would also move to weekly water changes around 10%. To get ahead of the issue, you’ll need to do large water changes on top of that as well. A 50% water change would bring your nitrates down to 20 and in a much better ball park. Siphon our as much gunk as you can during the water changes as well.