I answered a question on mold, and this one popped up in my head...
I have a concrete porch, that has a major algae problem. I have tried bleach and scrubbing with a stiff broom, and that gets rid of some, but not all. I have tried a couple of commercial algae/concrete cleaners, and the algae went "Ha, ha, ha-ha! Is that all you have?"
I want to clean the porch good this year, it is almost pool season, and I am about to put up my pool. What to do? Outside of dousing with gasoline and setting it on fire, that is.

Hi bigbopper

Have u tried an algaecide and power washer.

Have to put algaecide on and keep wet for about 1/2 hour. Then power wash off. May need more then one treatment.

Hope this helps

Good luck

Chuck