Of course it exists in the CS. If it didn't it couldn't possibly be in game!
The easiest way to find it is to go to underpall cave or wherever you find, open the landscape window and press ctrl+right mouse button. This (or something like that) is like an eye-dropper which selects for you whichever ground cover is under your mouse.
Does this method tell you which one you selected, or just make it the current one to paint with? It would be helpful if you could note it down for future reference.
With all due respect, I'd ask you not to give the guy hope as the texture he want just doesn't exist in the cs.
Things don't exist "in the CS", they are found by the CS in the game's Data folder and made available. If it's there in the game, it can be used in the CS. Some CS functions mean that you need to unpack an item from a BSA before you can do what you want, but that shouldn't be needed here. Since the grass is generated by the game engine, you won't see it in the render window, but the texture that tells it which grass to use
will be there.
See The Greatness' reply for the truth - if the CS doesn't have it, the game can't use it. It's possible for a mod to replace the bsa files and get rid of it completely, but then you'd be getting a different one in the game as well - and I don't think any of the retexturing mods go so far as to actually DELETE content, they just override it. When you start the CS, the only thing you select is the esp's, you get the meshes, textures and all the other data files from all the mods, so you can't even turn off a texture there, like you can for quests, scripts etc. Using BAIN or OBMM will move files in and out of the data folder, but the original bsa's (which contain this texture) aren't touched., so you can't lose it that way.