GHA,Phosphate, lighting

can someone explain how GHA grows. My phosphate is .02 and i understand there is phosphate in the water not detected. How is the GHA not dying off? I only feed 1 cube of frozen brine shrimp per day. and my corals and fish pretty much finishes it all off. I have Chaeto growing in my fuge as well. I wait a week hoping my algae goes away only to find it has increased. I dont want to do another 3 day blackout. I certainly do not overfeed. I watch videos and most of them say if I keep my nutrient import in check, it will die off. not happening at all. I think i have the only yellow tang that doesnt eat GHA lol.
any advice will be greatly appreciated.

I have a 93 gallon cubed tank

mag 1240
alk 9.8
cal 450
phos .02
ph 8.4
nitrates 0