Is this on the shower side or a dry area? If it's in the wet side, greenboard is a lousy material. National codes no longer allow it in a wet area, and regular drywall is fine in dry areas.
But, that aside, are you trying to save the tile, of just remove it?
It's usually nearly impossible to remove a tile intact if it was installed correctly. To get it off, first cut or saw or rout out the grout all the way around the tile you desire to remove. Then, take a hammer and a chisel, whack it in the middle, and then work out towards the edges with the chisel, chipping it out. Make sure you wear eye protection and long sleeves wouldn't hurt, as shards can be sharp depending on the type of tile.
If the greenboard is on the wet side, you'll likely find that it is deteriorating low down where it gets the most spray. Neither tile nor grout is waterproof, and with enough moisture on top, will eventually cause it to rot out.