What kind of wall material should be used in wet areas in a bathroom?
Greenboard (or any paper faced drywall) is an unacceptable substrate for any wet application (shower or tub stalls).  Regardless of tub stall surface (tile, fiberglass, etc) you will get some moisture penetration through the shower liner and this will eventually create bigger problems.

You need to install Hardibacker or Durock (or any cementitious board) as the substrate for your tub/shower stall.  Matter of fact, in many municipalities it is required by building code.  It only needs to be installed in the area of the tub/shower, though no need to extend further out.

The rest of the bathroom can be done in greenboard though (and that isn't a bad idea).