I've had mold on concrete and stone before, and my 200 year-old neighbor introduced me to the magic of bleach. Make a strong soultion and with a sponge or spray bottle soak the mold. Let it sit for a while and soak it again. You can tell if it's working because the mold will change color in some way. Kill it good (not just the surface) then scrub with a scrub brush. If it's alive, bleach kills it.

Muratic Acid works great you can get it a Home Depot