Ask This Old House (Published August 2013 Issue) - Q&A Section: How to get driveway mold under control?
To remove mold or mildew of the concrete you will need SealGreen Mold and Mildew Remover. This product is totally environmentally safe.and simple to apply.
Mix the product according to directions, apply with a garden pump sprayer soaking the surface to over saturation. Allow to rest for 10 minutes before pressure washing. For better results you can apply the cleaner a day before washing let it sit overnight and next day pressure wash the surface.
For a video of this process visit SealGreen Mold and Mildew Remover.