Alga and Fungus living together are known as Lichen.
Taking a closer look at a Lichen, it reminds me of the slime monsters in the black and white SciFi movies from long ago *shudder*
Lichens come in a variety of colors and forms, gray, brown, green, crusty, hairy, bubbly, etc, and can be found on every continent of our Earth in all climates and altitudes.
Lichens are not harmful to the plant, they just sit on the surface. I don't worry too much about them on the Crepe Myrtles since those trees shed their bark and with it the lichens.
To control the growth of Lichens try Copper fungicide.
Want to know more about Lichen? go to All about Lichen