My understanding is that it's definitely not okay to use Greenboard for a tile backing. From what I've read it MIGHT be okay if everything is done just so and the grout sealed, but only a very small minority claim this, virtually everyone else says concrete type backerboard.
My understanding also is that USG says the following:
Does this mean it should not be used as a backer for a glue up tub/shower surround?Panels are not intended for use in areas subject to constant moisture such as tub and shower enclosures
It's my understanding that greenboard was designed to be used in moisture areas like bathroom walls, but not specifically behind surrounds and/or tile.
If it makes a difference this is the greenboard from about fifteen years ago, not the newer type.