On 7/20/2014 7:30 PM, Dan Espen wrote:
buffie writes:

It's like green slime. You can power wash it off. It's not moss, or
algae, or ivy. We do live in the woods, but in a open area.

It's most likely an algae, especially since it's green.

There are slime molds but I have never seen or heard of one that
was green.

You might want to be more specific about where you live.
In the woods doesn't really tell the real experts here much.

OP need only google algae siding images to see.

I get it in shaded areas. Wash and bleach but it keeps coming back.