Wayne, from MSN Encarta, My bold,

Quote:

Algae vary greatly in size and grow in many diverse habitats. Microscopic algae, called phytoplankton, float or swim in lakes and oceans. Phytoplankton are so small that 1000 individuals could fit on the head of a pin (see Plankton). The largest forms of algae are seaweeds that stretch 100 m (300 ft) from the ocean bottom to the water’s surface. Although most algae grow in fresh water or seawater, they also grow on soil, trees, and animals, and even under or inside porous rocks, such as sandstone and limestone. Algae tolerate a wide range of temperatures and can be found growing in hot springs, on snow banks, or deep within polar ice.

I am not an expert on this and I didn't even stay at Holiday Inn Express, but this definition certainly leaves room for the stuff growing on roofs to be algae.
Jim

__________________
Jim Luttrall
Mr. Inspector.net, Inc.
Allen, Texas 75002