When you place a grass block on a layer of snow, the sides receive the snow texture, and the top turns a weird greyscale version (I'm guessing the default, and what it looks like under the snow), instead of just a regular grass block.
The issue is fixed if a block is placed on top it, or if it is moved by a piston.
- Place a layer of snow.
- Place a grass block on top of it (so that the snow is replaced by the grass block)
→ You now have a snowy grass block without snow on it