Most likely an algae, especially if green in color, Lichen Moss or other mosses would have more of a structure to them. All enjoy cooler temperatures, good levels of humidity and that goes along with shady environments.
There are commercial sprays that can be applied that will manage the algae. These often kill the algae by rupturing cell walls, which can then be rinsed away. Be sure to read and follow directions and make sure it it safe for blacktop and and surrounding surfaces, plants and dirt where the rinsed water solution will flow.
Your local hardware store would be a good starting point, perhaps a pool supply store as well.
Cheers, Richard
