Dark stains on stone

[Konrad]

This is a house in a buddy's neighborhood. We don't have very many stone houses like this, but the ones that I see often have this dark coloring on them. Is it a bacteria or fungus or something? What would take it off? Is there any reason it only develps on the east and west elevations of the house, but not the north or south? These stains in this photo are the west elevation.

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[Vince Santos]

Could that be Efflorescence?

[charlie]

I see this all the time in the NW---especially on Sandstone. It is most likely a lichen and I suspect that it shows up where there isn't an overhang or other source of moisture.