What is it #3, stump the home inspectors

Lichen,  I think they make a special soap that you spray on, not with a pressure washer.  I have seen homes that are under or near pines and old oaks that have the soap sprayed on and it look like a new roof after the process.

It's algae!  What do I win??  :)                                                                                                                                 

Ha!  Now that I have all of the answers I can safely state 'algae!"  I have never had the happy occasion to see it... so I guess I have that joy to come..

It looks like paint on a shingle, but if it's on the north it is mildew. Pick up the house and turn it to the east or west. Correct solution: mix a ratio of chlorine and water mixture 6-7 to 1, let sit, and spray off with garden sprayer.

There is always something to find during a home inspection like the photo above. Best to take care of it and move on

I think it's algae.  I see a lot of homes with that issue.  Is there a problem with it and should we be concerned about it?

Pigeons?

Love AR , we have experts on everything! 

As cold as our weather has been the last 48 hours, all I see is ICE. We definitely don't see algae here in Northern Colorado much so I'll chalk that up to one of the few things that aren't on our list of 1,000.

-Dee

I'm going to go with lichen.  It can also be found on trees and rocks.  I'm hoping that is not how it got there.

Living in the Pacific Northwest, where we have all kinds of algae, moss and lichens, I am going to agree with Jay (#6) and Gary (#16).   I believe Jay's remedy will do best.  Not sure if copper or galvanized strip (zinc) would do best to prevent re-occurance.

   Although moss is said to grow on the north side of a tree, what you show in your pictures, as Jay said, appears to be lichens.  However, those commentors who said "algae" are partially correct in that algae is a component of lichens.  Mosses are plants and it's definitely not mildew/mold.  See the pictures near the end of this Wikipedia page:  http://en.wikipedia.org/wiki/Lichen  See the similarities to your picture?

   eHow has an article on How To Kill Roof Lichens at:  http://www.ehow.com/how_8128582_kill-roof-lichens.html

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Lichen would be correct, and pressure washing is the LAST thing you would do to a composite shingled roof! You will blow all the protective granules off of the shingles and reduce the life of the shingles. 

You're gonna have to try harder than that to stump us :-)

Kenneth, it looked like Pigeon droppings at first. Then again I don't think they care which side of the roof they do their business.

Kenneth, I'm not an inspector, but I've been on a bunch of roofs. I'm gonna say it's fungus or mold. I get it on the north side of my roof, and it's blocked my the second story. Lots of shade.

Kenneth - Isn't it moss that grows on the north side of the tree.