It's probably algae or moss, rather than mold. I use a pressure washer to clean it off. Actually, I hire
someone with a pressure washer to clean it off my patio, patio furniture, plant containers, sidewalks, exterior house walls, etc. The stuff really is everywhere.
It doesn't look like terribly hard work, but I don't own a pressure washer, nor do I want one. I know I could rent one, but the guy next door already has the equipment and doesn't charge me all that much. I usually have it done every other year. To keep the place pristine would require having it done every year, but I'm not that concerned about it.
That doesn't mean you shouldn't clean your
place every year. It's just that, in Oregon, moss and algae cover almost every surface in sight. I guess I'm used to the sight of it, and it doesnt really bother me. We joke about moss growing behind our ears and between our toes.
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams