Flooring Tile - Leave Greenboard And Reuse?

So, I've started my guest bath remodel project. This afternoon I pulled down all of the shower tiles and I found that when the home was built, they used greenboard for the shower walls.

I purchased some 4 mil poly liner and 1/2" cement backer board and I'm going to pull down the greenboard and do it right. However, I have a problem. The rear wall of my shower has a pocket door behind it, which obviously means there are no wood studs. Tapping the greenboard surface, back and forth, I can't find any kind of framework that the the board is attached to. I'm not certain how this piece is secured.

So my question is, would it be OK to leave the greenboard on that rear wall intact (it is in very good condition) and just prep the surface with some thinset to level it out? It's a 5' long tub/shower so the majority of the surface should not get wet during most showers. Also, this is just our guest bath, so it does not get used regularly.

What would the experienced people here suggest? :)

Thanks.

Lee

Rip it out and make sure there's framing for the new cementboard, add it if needed.:)

The pocket door framing goes horizontaly, usally 1x4. Just be careful when screwing the cement board, not to use screws that are too long and will interfier with opperation of the door.

Thanks for the responses; I guess I'll just take it down carefully and see what's back there.

close the door first so it is not in the pocket at the time of demolition.