I'll get dirty so you don't have to! This is called surface cleaning. Here we are using a 20" surface cleaner that runs off of our commercial pressure washer. In an instance like this, we pre-treat the surface with our proprietary algecide solution, and allow it to "dwell" for about 10 to 20 minutes prior to surface cleaning. This process is far superior to just pressure washing alone, because it will actually kill the organic growth before removing it, assuring that it will not be able grow back.