Physcia tenella (Scop.) DC.
A very small, foliose lichen, with long marginal cilia and usually with terminal soralia on the turned up undersides of the mature lobe-tips. The lobe surfaces often dotted with black pycnidia. Short-stalked apothecia with with blue-black or thinly pruinose discs. A common indicator of nitrogen enrichment and found on trees.
Smith et al. editors (2009) The Lichens of Great Britain and Ireland
Dobson (2018) Lichens: an Illustrated Guide to the British and Irish Species
Ahti et al. editors (2002) Nordic Lichen Flora, Volume 2: Phyciaceae