They may look scary, but lichens are harmless organisms that consist of a fungus and a green or blue-green alga, which live in association with one another to look like a single plant.
Lichens often appear leafy or crusty and are colored gray, green, yellow, or white.
Typically they can be found on dead or declining trees, on rocks, or on the ground. Their presence on failing trees is a sign but never the cause of poor plant health. Less vigorous plants tend to have a more open canopy, which increases sunlight penetration and subsequent lichen growth. Controls are not necessary. You can remove the lichens from your plant with a stiff brush, but they will probably reappear if you do not determine the cause of decline and correct it.