I used white vinegar and a hard bristled brush to remove mold and lichen from my patio. I had to rinse everything with hot water several times.
Try wet & forget outdoor. It removes the green & black over time. It's safe for pets & should be good. I bought mine at Lowe's. They also sell it at walmart.com & qvc (they have a good video).
Hi George, here is a link to some remedies for that. It says it is for lava rock but I am sure it will work on any rock.