The key to getting rid of a lichen is locating the real problem and correcting it. Once that's done, the lichen will go away on its own. Lichens not only grow on trees, but on solid rock as well. Some people even select lichen-covered rocks to put in their gardens or landscapes because of their pretty colors. Lichens can be grey, brown, rust, or even greenish.
Lichens grow very, very slowly.
They might only grow 0.1-10 millimeters per year.