I just wasted about 2 Hours using a product by ZAP called concrete cleaner. following the direction it did nothing.

so i went back to doing what i did last year.

Bleach. sprayed it on with a houshold spray bottle. using a good bristle broom i worked it into the concrete then power washed it off.

good as new
also since its bleach you dont have to worry about it getting in the pool.