Possible Verrucaria ?

I have a suggestion, but it is not a confirmed identification from your photograph. Verrucaria squamulosa has greenish squamules set on a black basal layer. Curiously, the black basal layer is not mentioned in the type description, or subsequent literature despite, in my experience, it being a consistent feature and one of the more conspicuous features. This makes it look rather like V. nigrescens but with subsquamulose greenish algal units. Perithecia are usually present and these are often isolated on the black basal layer, the squamules either not touching the perithecia or sometimes partially overlapping the perithecia. It gives the fanciful impression of having been built in stages, a black basal layer laid down, a scatter of perithecia added, and then a variable density of greenish ‘squamules’ laid on.

[attachment=0]Verrucaria squamulosa Sawst.jpg[/attachment]

More details and images here:

https://fungi.myspecies.info/all-fungi/verrucaria-squamulosa

V. squamulosa was first recognised as a distinct species in 2003, thought to be closely related to V. macrostoma and sometimes confused with that species but, due to the black basal layer also probably confused with V. nigrescens.