Lecanactis abietina

Lecanactis abietina, the old-wood lichen, is a species of leprose lichen in the family Roccellaceae. It forms a thin, mauve-grey to whitish-grey crust on the bark of both deciduous and coniferous trees, typically in shaded, humid habitats, and is regarded as characteristic of old-growth forests in cool temperate regions. The species is widespread in northern and central Europe and also occurs in North America, Australasia, and Papua New Guinea. Its nomenclatural history played a central role in the conservation of the genus Lecanactis, for which it serves as the type species....read more on Wikipedia.