There are 1800 species of lichens in Britain, most of which have no common English names, and over 600 mosses and although I think I have identified three of these lichens which incidentally do not harm the trees, I cannot be certain but perhaps someone can name them. They grow very slowly and are sensitive indicators of air pollution with more varieties now growing in areas which were once heavily polluted.  Apart from having been used for dyeing cloth and wool and even perfume, lichens provide food for some creatures, and shelter for insects while birds sometimes gather lichen or moss for making nests