Should I use cement board or green board behind the walls of a neo angle shower?
I would always use cement board in the shower area.  I would extend the cement board to where you shower unit will cover.  You can go past that point just plaster with a nice joint compound, like durabond to join to the rest of the plaster board for a permanent bond. I typically do not use green board unless you expect the wall can get wet, like a basement.  My bathroom is upstairs and my house would wash away before those walls got wet, therefore regular board is fine.