Thelotrema lepadinum

About

Thelotrema lepadinum is a species of crustose lichen in the family Graphidaceae. Commonly known as bark barnacles, it forms a greyish crust on tree bark, dotted with numerous small, urn-shaped fruiting bodies that have distinctive white-rimmed openings. The lichen is found worldwide on smooth-barked deciduous trees, and in Europe it serves as an indicator of ancient woodland due to its preference for long-undisturbed forests. First described by the Swedish lichenologist Erik Acharius in 1799, the species has been reclassified multiple times throughout its taxonomic history.

Summary from Wikipedia