The greenboard on the right side of the shower stall is cut out for the shampoo holder cubby boxes that are framed in. That's some pretty fancy cutting to be able to leave that thin strip between the two boxes. If you've ever done anything with gypsum sheetrock, you know how fragile it can be.
This greenboard sheetrock is the first layer of moisture barrier protecting the rot prone wood framing behind. Tile doesn't get installed directly onto this greenboard - several more layers will be built up before the tile is installed and everything is done.