The blotches are crustose lichens (the somewhat orange patch seen magnified below).  Less obvious blackened sidewalk surface (seen at top left of the lichen) harbors algae!  The image at right (taken at 400x) illustrates some of the cyanophyte algae found on the dry sidewalk.


On a dour note, such growths (which can be quite slippery when wet) have undoubtedly contributed to the circumstances leading to many a broken hip in the elderly.  Slimy algae coating rocks in streams have been acknowledged for their role in more serious mishaps where people have slipped and fallen to their deaths from the tops of waterfalls.