Thanks for your question. From what I can see in the photos, I think what you're seeing may be lichen. Key points about lichen:

• Lichen are non-destructive, living organisms composed of a fungus and algae living in a symbiotic relationship. In a symbiotic relationship, two organisms function in a way that is mutually beneficial. Because the algae derive nutrients through photosynthesis, and the fungi protect the algae from drying out, lichen can live and grow in extremely barren areas.
• They grow in colonies on tree trunks, rocks, and fences, even in Antarctica.
• Lichen grows in many interesting forms. Those with a flattened and crusty appearance are called crustose lichen. Foliose lichen have raised, leaf-like lobes. Fruticose lichen have branched growths with finger or thread-like projections.
• Colors range from gray-green to bright orange-red.
• Lichen are often blamed for killing a tree or shrub but this is not true. They do, however, grow on slow-growing and sometimes declining trees and shrubs.

Reference: Lichen, Algae, and Moss on Trees | University of Maryland Extension (umd.edu)

The referenced article provides information that will help you identify the presence of lichen. You may also want to consult an arborist.

Tina

Durham County Extension Master Gardener volunteer