Once upon a time in 2018, I went to England and was struck by just how much moss there is on everything (seriously, everything single thing has a coat of green fuzz if it's not routinely pressure washed). I also decided that England is much like Skyrim, and therefore, Skyrim needs more moss.

When I came across tamu75's Moss shader resource, I set about applying it to...just about everything, really. This makes Skyrim a far mossier place. You may actually find it slightly monotonous at times, as the shader doesn't play as nicely on large, flat objects as it does really lumpy ones. Because it's a shader, it can be applied to any mesh that goes by the vanilla names. Got a great mesh replacement, like Xenophobe3's Stone Walls? This shader will put moss on them just like the vanilla stone walls (and because that mesh is lumpier, the shader is even nicer!).

Due to being a shader, it can only be applied from one direction uniformly, rather than worked into nooks and crannies from all angles like a well-made texture (for example, I didn't put it on the farm houses because it just looks bad on the roofs; Moss Covered Roofs does it much better), and if it covers part of the top of a mesh, it covers all of the top (thus not applying it to roads or bridges - while it makes sense the edges would be mossy, the middle where people are constantly walking would be worn away; that would have to be done with a texture, not a shader). It also doesn't respect overhangs, meaning that covered walkways and bridges are out - it looks fine on the roof, but absurd all over the actual walking area. This is mostly applied to rocks and a few other natural or less-disturbed features, rather than large swathes of human habitation - but because Skyrim has a lot of rocks, this makes a drastic difference in the landscape.

Another limitation of being a shader applied to static objects: only one can be applied at a time, and as snow is also a shader, you can't have fresh-fallen snow on top of moss (or, if you can, I don't know how; the Creation Kit doesn't make it immediately obvious, and I'm no modding wizard). Also, because it's applied to every copy of an object, you can end up with absurd situations: the devs put a bare rock in the middle of a snowy field, rather than a snowy rock, and because that rock used to be bare, it now has moss (well, moss does grow just about any time there's enough liquid water, so if it sometimes thaws even a little bit...). That same previously-bare-but-now-mossy rock may also be used a dozen times in a forest, where it looks perfectly sensible covered in moss, but you can't pick and choose which copies of that rock get moss and which don't; it's all or nothing.

Installation is simple: just use Mod Organizer or drop the .esp in your Data folder and activate it. Because it's script-free, it should be safe to install or uninstall at any time, but as with all things Skyrim, I'm not taking responsibility if removing it mid-game eats your save file; keep a backup from before you install it.

Try it with these other mods:
Total Tundra Texture Transplant
Mossy Floor
New Grass for Whiterun
Green LODs.
HiRes Autumn Forest Grass and Green Grass
Natural Aspen Recolor
SkyeMoss
Dandelion Seeds
Enhanced Wetness and Puddles