There are gray-green patches on the trunk of my tree. What are they? Are they harming the tree?

FAQ
Question: 

There are gray-green patches on the trunk of my tree. What are they? Are they harming the tree?

Answer: 

The gray-green patches are probably lichens. Lichens are unusual organisms. They consist of two unrelated organisms, an alga and a fungus. These two components exist together and behave as a single organism. The agla provides food via photosynthesis. The fungus obtains water and minerals for itself and the alga.

Lichens are common on trees because the bark provides a suitable place to gather sunlight and grow. They grow especially well on dead branches because they receive more sunlight. In addition to growing on the trunks and branches of trees, lichens can be found on exposed soil surfaces, rocks, wooden fence posts, shingles, gravestones, stone walls, and other sunny surfaces. Lichens may be flat, leafy, or branched and hair-like. The lichens on trees are often gray-green. Other species may be orange, yellow, slate blue, or black.

Lichens are fascinating, unique organisms. They do not harm trees.