I tested the pool on Friday - everything was all in balance. Why do I still get green algae on the north facing wall of my pool? I've cleaned my cartridge filter twice this month hoping that would help. I keep it brushed off as soon as I see it. I have natural rock that touches the water on that side of the pool. Could there be some sort of algae that is seeping out of the rocks? The green starts at the surface of the water then grows downward. Water is crystal clear and beautiful. I'm using the last of my tri-clor tablets then going to the bleach method.