See:
Hazelnut (Corylus avellana) - Mosses and Lichens
Algae, Lichens, and Mosses on Plants
Notes Moss and lichens grow profusely on trunks, limbs, and twigs of many trees and shrubs. The cool, cloudy, wet weather in western Washington and Oregon is ideal for these organisms. Mosses are green plants that have a complex structure that resembles stems and leaves. Lichens have two components-a fungus and an alga living in association with one another to give the appearance of a single plant. These primitive plants contain chlorophyll and make their own food, so they do not directly injure the plants on which they grow. Reduce wet conditions that are favorable for growth by opening up the canopy to allow better air circulation and light penetration. Application of copper-based materials in the fall will help provide some control.