Its probably an algae which is much more common in the summer months. There are a few products that will deal with this and no doubt others on here will advise. I tested a product last year that nothing will stick too and is being tested on river signs and banks, lampposts etc with brilliant results.
I sprayed all our paths and drives and boat with it last winter which would have normally been plagued with algae throughout the year, and there isn’t a sign of the algae returning with the exception of the traffic lane to the front door (where it has eventually worn off) This is the ideal candidate for all roofs as well.
The idea is to market it, but posting chemicals can be expensive as everybody knows.
I will be looking at the costings in the next few weeks, maybe I can get some samples out.
