I believe that this gravestone demonstrates a variety of lichen including crustose lichen.As the gravestone is close to the River Wye this would seem feasiable.
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.
Lat/Lng: 51.7035, -2.6795
OS grid ref: SO531007
Graveyeard between River Wye and main Monmouth - Chepstow road