Why is my pool still green even though chlorine levels test high?

Probably too much cyanuric acid (CYA), also called stabilizer or conditioner. It binds to chlorine and makes it less effective, even when test strips show high levels. Once CYA goes above 50–60 ppm, you need much higher free chlorine to actually kill algae. Check your CYA level — if it’s over 100, drain part of the pool or use a CYA reducer, then shock properly based on your CYA level.