Lichen

Many people become concerned when they see lichen growing on their trees, wondering if their tree has a disease. However, lichen, like moss, is not a plant parasite. Instead, the lichen is using the structure of the tree for physical support, but is not tapping the tree for its nutrients.

Lichen and moss hitching a ride on a Rhododendron

Lichen is a composite organism, which consists of a fungus and a photosynthetic partner (such as green alga or cyanobacterium) growing together in a symbiotic relationship. This organism produces its own food via photosynthesis, and absorbs moisture from the environment.

Lichen is a composite organism, but not a plant parasite

The Integrated Pest Management (IPM) site indicates that you can use herbicidal soaps to control mosses, lichens, and algae. But, personally, I like the look of lichen and moss growing naturally on trees.