The ability to withstand drying-out has been an important challenge that plants have overcome during the course of evolution. Plants evolved in water and the colonization of the land involved the evolution of characteristics to help them avoid desiccation.
The green colour on the north face of trees and walls is caused by the growth of a tiny green plant called Pleurococcus. Pleurococcus is an alga, the closest relatives of which grow in lakes and rivers, yet it withstands the drying effects of exposure to the air and, during its lifetime, is never fully immersed in water.
When a colonising Pleurococcus lands on a surface, it releases a carbohydrate-rich slime that sticks the cell in place and stops it being washed away. Once safely attached this pioneering cell reproduces by cell division (mitosis), where each cell makes an identical copy of itself and colony growth begins.
Since each of the new Pleurococcus cells release sticky slime, the colony soon sits on extracellular glue that not only attaches the ever-expanding colony to its substrate but also forms a matrix that can be colonized by other organisms, such as bacteria. Soon this matrix becomes a complex environment, with a large array of different chemical substances rich in a variety of algae and bacteria.
These sheets of diverse microorganisms, suspended in a mass of carbohydrate-rich chemicals, are called biofilms. The ability to form biofilms allows Pleurococcus, whose closest relatives live in the water, to survive on the dry and exposed surfaces on the land environment - the air.
The ability of algae to grow on land is likely to be a very ancient characteristic. There is fossil evidence that shows that mats of algae existed as far back as the Proterozoic, the geological eon that ended 540 million years ago. These mats would have had the characteristics of biofilms, similar to those formed by Pleurococcus today.
The green hue that you see on damp, exposed surfaces is therefore a reminder of a process that occurred many times in Earth history - colonization of the land by life.
Many organisms left the water and got a foothold on the dry, continental surfaces of the planet. Some diversified once they got a foothold on the land, giving rise to a seemingly endless diversity of forms. Others, like the ancestor of Pleurococcus, have changed very little since they first left the water over half a billion years ago.
Graham JE et al 2009. Algae. Benjamin Cummings.
Guiry MD, Guiry GM 2014. AlgaeBase. World-wide electronic publication. National University of Ireland, Galway.