Pressure cleaning should not be just a spray and go procedure. The cleaning should focus not only on the appearances but also sanitize the bacteria infested area. When it comes to anything relating to mold the surface should be sanitized almost to eat off of. Of course we do not mean this in the literal sense but should understand that mold spores travel through the air and can be inhaled. Prolonged exposure to any mold can cause health concerns.
Similiar to every cleaning and sanitizing it is important to use the best tools and chemicals to eat off any bacteria on its surface. Exterior surfaces often collect mildew deep in the floors pores that are best when blasted with highly pressurized water, where the bacteria has the only option to shoot out and loosen on to the ground.
Please be sure to rinse away as soon as possible because prolonged exposure to the sun may cause the dirt to seep back into the stone and will take longer back breaking work to push away again compared to when it is loose and wet on the surfaces top. After and preferably before use a strong mixture of chlorine to away the mildew and neutralize it to nothing.