Your chlorine is probably locked up by too much stabilizer.

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.

Most pool test kits measure total chlorine, not active chlorine. Once CYA goes above 50–60 ppm, you need much higher free chlorine to actually kill algae. At 100 ppm CYA, your “high” chlorine might be doing almost nothing. The algae just shrugs.

Other possibilities: phosphates from fertilizers, mustard algae that resists chlorine, or low pH. But CYA is the most common culprit when people swear they have enough chlorine and the water is still green.

Check your CYA level. If it’s over 100, you can either drain part of the pool and refill, or use a product that breaks down CYA (slow and expensive). Then shock properly based on your CYA level.

That green is algae laughing at your test kit.