Closer inspection of the wall shows that in the some sections, the mortar has clearly been repointed. I suppose that's obvious. Now it is not clear that siliceous-loving lichen would want to grow across lime-rich mortar but it clearly has in some places. It may be that the repointing has removed some lichen. There are clearly some patches that are stone but differently coloured suggesting that they were covered at one point.
Not a problem but an interesting observation is that the slight overhang of the capstones keeps the top section drier which may affect the growth of lichen. In places the bottom of the wall is mossy because that is the wettest part.
