Fungus on Apple Tree
This appears to be a lichen.
Lichens are a plant composed of a fungus, an alga, and quite commonly, a yeast that live together in the same body. Lichens often appear as green to gray-green leafy or crusty growths on the trunks or branches of plants. They are not harmful to the plant, and are merely using the plant as a place to anchor.
Treatment: Controls are not necessary, since lichens are not harming the plant. Lichens will gradually disappear if plant health is restored. Remember they will probably reappear if you do not correct the true cause of the plant’s decline.