That will work but sounds tedious. Can you put bleach and water in a spray bottle and just carefully spritz them?

As long as these rocks aren't near grass or other vegetation, go the bleach/water route and scrub a bit as you are spritzing. Then hose off.

What you probably have is moss. If this area of your yard is shady, it's most likely going to grow back.

Is moss just green or can it be black, too? I just assumed it was mold because its black with a fuzzy appearance. Yuck!

I just finished pulling some weeds that were growing in this rock bed and I'm going to start spritzing the rocks with the bleach and water in a spray bottle.

thanks!