So I've tried various mold and algae killers to get rid of moldy concrete. I've tried scrubbing it, and I've also tried pressure washing flat round mold from my concrete side walk with no success.
Ultimately what appears to have worked after about a year was Wet & Forget .75 Gallon Moss, Mold, Mildew & Algae Stain Remover.
Last spring I bought Wet & Forget and sprayed my concrete sidewalk. I didn't see any real noticeable results, but did think some of the mold spots get less. So a few months later I sprayed again, and I still wasn't sure it was working. I may have sprayed once more before the winter. Well it turns out about 1 year later, I can look and see almost all the moldy / algae spots are completely gone and my concrete sidewalk looks like new again. So it works, but just takes time for it to work. They even state on the Wet & Forget container that it may take 6 months to see results. NOTE: Winter / Snow / shovelling may have helped to finally get rid of the dead mold / algae from my concrete sidewalk after using Wet and Forget.
I also spray it on my deck and deck chairs and table. It works really well getting rid of green algae from the chairs and worked way faster (within a day or two on my chairs). So I would recommend giving Wet & Forget a try, but don't expect results right away.