Lichen

The presence of lichen tells you that the shrub or tree is not growing as well as it should be. Suggest you prune or rub off the lichen and prune the rest of the tree if you have not recently, then fertilize in spring and again in summer this year to stimulate new growth.
Lichen (pronounced äóìlikenäó) is a biological class of complex organisms (a fungus and alga or cyanobacterium) that have a symbiotic relationship. The green algae and cyanobacteria produce food by photosynthesis. The fungi cannot photosynthesize, but provide a protective exterior surface for the algae/cyanobacteria. This enables the algae/cyanobacteria to exist in full sun, thus maximizing its ability to produce food for both. Lichen gain water and mineral nutrients mainly from the atmosphere, through rain and dust. THis is not a disease and will not hurt the tree or shrub.

Lichen not only grow on plants but on the ground and on rocks. Lichens have a great deal of variability in color and form depending on the species. The color can range from white to green to yellow/orange/red, and some have black specks. Some are flat and others look like ruffles. Some look like small, leafless shrubs.