Nonsense, if you have an exhaust fan or window, use regular drywall and prime it and paint it or wallpaper it, it's sealed then, greenboard is a waste of money, always use cementboards in wet areas.:)
You can not tile over the plywood, you need a backerboard and maybe some more plywood first, but from reading your other thread, I won't waste my time asking what you have for a subfloor.:eek: