First and Foremost, Lichens are completely harmless organisms that just happen to find tree bark the ideal place to grow on.

Lichens are made up of a combination of fungus and an algae ( actually its not a true algae but rather a cyanobacteria - but to keep this simple and for all intensive purposes, it's like algae) that benefit from living together.  The fungus provides the nutrients from the air (dust) and from the outer bark of the tree ( don't worry, its the outer bark that's dead and it doesn't hurt the tree at all) and holds moisture from dew and rainfall.  The Algae in return produces food for them both through photosynthesis. Lichens only grow when there is sufficient water so in the dry parts of summer it kind of hibernates and waits for the next dew or rainfall.  When they do grow, its pretty slow ( often like millimeters a year) .

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Lichens  Come in varying colours  - mostly grey, but can be orange, green, yellow and even red.  Lichens, very often can completely  cover the bark, but as noted earlier, it is harmless.

 There is absolutely no reason to remove or control  lichens living on your tree. It's Harmless

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