Lichen

Thank you Maria dB and Gilma.

Maria dB: Here is the answer: lichens are combinations of green algae and fungal tissue. Each lichen specie is one alga species plus one fungus. The alga, a microscopic green plant, makes the food for the duo, while the fungus, a non-green plant, gives the pair support and soaks up moisture. This cooperative arrangement benefits both creatures and the surrounding system, but raises a frequently asked question...Are lichens harmful to your trees? No. Those grayish-green patches, usually one to three inches in diameter, are not feeding on your trees. Since lichens are able to make their own food with moisture and sunlight, they have no need to parasitize other plants. Lichens grow on the surface of your tree, and do not penetrate any tissue. Instead, they make use of the trunk or branches for support. They do not cause plant diseases either, with one exception: in certain wet, tropical areas, lichens have grown in such thick layers over trees that their shade alone has caused leaves to die. And these information from attached website: http://www.savatree.com/are-lichen-harmful.html