Leprocaulon calcicola

A distinctive lichen in appearance and habitat; a bluish green leprose thallus lacking a true medulla, without projecting hyphae and growing on calcareous walls. With zeorin and usnic acid (K-, C-, KC+ yellow, Pd-). The KC+ yellow reaction distinguishes L. calcicola from other leprose British species, except for Lepraria ecorticata on acid rock and an un determined species found in crevices on dry bark on old trees, both of  which however haves a distinct pale yellow tinge. So far mainly found in East Anglica, but likely to be found elsewhere in the lowlands.

See Fungi of Great Britain and Ireland & Orange et al (2017) The Lichenologist 49: 183 - 188.