Jimmy, that is a very interesting question :eusa_thin
The algae growth that you are finding on the side of your shop is a product of shade keeping the surface damper that the ones that are exposed to more sun. Usually you find this on the north side of the house. It receives the least amount of drying sun exposure.
When air drying wood, air movement is your friend. It will carry away the moisture laden air created by the evaporation of the moisture in the wood trying to reach an equilibrium with the MC of the air around it.
When air drying wood, I don't think it is best to have it exposed to a lot of direct sun exposure. Because the sun will only effect portions of the stack, you will get uneven drying. Also trying to get even sun exposure to a stack of wood would mean that it will be exposed on all sides, this would leave it open to the re-wetting effect of rain.
I think the best way to air dry wood is to keep it in an area that has good air flow through-out the stack and overhead protection from rain.
Air flow across this planet is generally in a east/west direction as opposed to a north/south direction. So you would be best to place your stack so the broad side is perpendicular to east and west. If your area affords you that orientation then it is probably a good space to store your wood while it air dries. But if the building is blocking the air flow, like a north-side wall would then you will probably have problems with algae/mold growth.
The good new is that for the most part that stuff won't have much of an effect on the wood once it is dried and milled to it's final size. Most of that type of growth is only going to be on the surface, and will only be viable while there is enough moisture for it to growth on/with.
MTCW,
Dave
