Caution: Do NOT use iSpot to identify fungi to eat!
Some fungi are very poisonous so a mistaken ID could have serious consequences.
No interactions present.
I think your first thought is along the right lines - a dried algal or cyanobacterial mat. The pale green species overgrowing it in the first photo is probably a lichen. It reminds me of Agonimia tristicula, which I have seen overgrowing free-living Nostoc colonies.
Lat/Lng: 55.9176, -5.4248
OS grid ref: NR860746