You can tile over greenboard sheetrock whether it is in the shower or any other location. To do this, roll on a layer of sealing primer and leave it to dry for 24 hours. After that, you can tile over the greenboard sheetrock in the same manner as you would with any other bathroom tiling. First, apply the tile adhesive to the wall in small areas. Then apply tiles to this area, putting spaces in between the tiles. Repeat the previous 2 steps until the tiling is done. When it is all dry, remember to grout the tiles to keep them from getting water damaged.