Algae is using up your nitrates and phosphates. It uses those, along with light, to make food for itself. Without those things, it would die. Having any algae is an easy and reliable test for nutrients. (Just doesnt tell you have many ppm
You need to track down the nutrients door way into your tank.
Since you are battling algae and doing a 15% PWC
weekly, you are likely over feeding, or using bad food.
Another thing to check is your lights.
1)On too long
2)Old bulbs color shifting
I wonder if dying algae will release nitrates and phosphates directly into the water column? Maybe you are pulling the algae out and letting it drift around?