Nasty little green spots appearing on the surface of your concrete may be algae growing. Algae usually grows in wet areas that do not have good air circulation. Fertiliser from the lawn washing up onto the concrete can exacerbate the problem by giving algae extra nutrients to grow. You can clean and remove most algae stains from concrete with a little work and elbow grease. The right tools can make this job one easy enough to complete in an afternoon.
Apply chlorine bleach directly to the algae.
Sweep the concrete with a stiff broom brush to remove as much of the algae from the concrete as possible.
Attach your outdoor water supply to the inlet connection for the pressure washer. Turn on the machine.
Spray the algae stains with the pressure washer to remove as much as possible. Hold the nozzle six inches away from the surface, adjusting how close you hold the nozzle according to how tough the stain is to remove.