If all the maps in the game official as non official show the same grass type I'd suggest you trying to reinstall and see if it fixes itself that way.
However, if it's your maps only, you can always or should be able set the first texture as you please. If this works aswell. It's clearly something wrong with the mask picture itself.
If we get it narrowed down to that, lets focus on just getting any mask to work
A suggestion would be if you imported the watermill mask picture into your map folder. And then try to load that one, if it loads correctly and lets you set the 2nd or third mask then it's most likely wrong with the picture you made earlier.
If it lets you load the picture, but nothing happens when you change the 2nd and third textures, it's probably something with the game itself or the computer you're running the game on, first suggestion most likely.
If you aren't able to load that picture at all, make sure you enter the correct name, paste the picture in the right place n such things.
