Sounds like Fungi called Lichen

[Giddaymate]

Hi,
The best way to get rid of Lichen is to spray with house hold bleach or if you have a pool you can use your liquid chlorine.

There is also a product called “30 seconds” which is an outdoor cleaner for cleaning Timder, concrete that would be perfect for your job. It kills Algae, moss, mold, mildew and lichen which is very common on roofs.

I have used it myself. Just pour some of the 30 seconds into a pump spray bottle and spay all over your roof and your roof will look like new again. Here is there site to visit http://www.30seconds.net.

Here is more information about Lichen.

http://www.bugs.bio.usyd.edu.au/Mycology/Plant_Interactions/Lichen/lichenBiology.shtml

The site below mentions that Lichen is a good environmental indicator because it absorbs pollutants from the air ,well as stabilising the soil surface and Lichens have also been used as a food source during famines, and dyes by the Navaho indians, and as drugs for the treatment of lung and skin disorders. Reindeer feed on lichens.

http://www.burkesbackyard.com.au/1998/archives/26?p=2899

Regards Noelene