Some of the mossy plants in the Reach do not render correctly in foggy conditions (possibly in other conditions also, but most obviously in fog).
The mod listed below has identified the following meshes as problematic (though the mod simply removes them rather than fixing the issue):
Data/meshes/landscape/plants/
ReachMountainMossCover07.nif
ReachMountainMossCover08.nif
ReachMountainMossCover09.nif
ReachMountainMossCover10.nif
ReachMountainMossCover11.nif
(https://www.nexusmods.com/skyrimspecialedition/mods/65896?tab=description)