Verrucaria muralis Ach.
Thin, pale, often greenish thallus with black perithecia. More frequent on base-rich substrate but occasionally found on siliceous rock where the thallus is generally thinner
Smith, C.W. et al. editors (2009) The Lichens of Great Britain and Irelandd
Dobson, F.S. (2018) Lichens: an Illustrated Guide to the British and Irish Species