I've had this on my grout and deck corners since I moved in in June. When the water level goes up due to rain, it goes away. When I lower it, it comes back. I've scrubbed with bleach and a scouring pad (even removing grout in the process), and it comes back. I've never (knock on wood) had an algae problem in the pool, even when I was on tabs and accidentally let the FC hit 0.
Regardless of what it is, any bright ideas on how to kill it for good? Could I apply ployquat directly?
Other than this, the pool is consistently sparkling via TFP method, with minimal bleach use (about 0.7ppm a day) and passing OCLT.