Hallo Robin,

to me (as a lichenologist) the ascomata look like to belong to a saprophytic fungus

the lichen seems to be (almost) dead overgrown in some parts by colonies of epiphytic green (non-symbiotic) algae

what suggests the branch (and the lichen) is dead for relatively long period

lichenicoles are also usually much smaller

I must say that I am not the expert at all but tried to pass through some literature and older discussion on this forum

one of the candidates could be Dothidotthia ramulicola ?

discussed on the forum 6 years ago: see: "

http://www.ascofrance.com/forum/26773/didymosphaeria

the discussed specimen is at least seemingly slightly similar to yours, which could be immature? as noted in the discussion by Walter Jaklitsch for this taxon?

but maybe I am completely wrong, better to ask an expert on bitunicate pyrenomycetes

according to the description in the monograph by Barr (1989) in Mycotaxon this species should form gregarious ascomata on a stroma

perhaps the mentioned taxon is a dustbin name ?

all the best

Zdenek