Ecstatica's post in Cave wall glowing texture was marked as the answer
Thanks to your help and also the mod author, I narrowed it down.
It is a texture found in a few Falmer caves.
There is even mention of it on the STEP forums:
https://forum.step-project.com/topic/4045-falmer-glow-retexture-by-aiyen/?hl=%2Bshimmermist&do=findComment&comment=64457
and a mod that reduces the glow on Nexus:
https://www.nexusmods.com/skyrim/mods/35857/?
I would have thought something in all the STEP texture mods might have covered this - I wonder which ones are likely to have?
Maybe it's just too obscure.
In any case, now I know, I can try altering the texture /glow to my taste.
Thanks for your help!