Does the green board go all the way to the floor behind the tub? The flange of the tub has a 1/8 gap which would make it seem like the marble would have to curve out to cover the flange.what is the problem? Space out of square? How do we fix?

Which tube did you buy? The flange is nailed directly to the studs and then your board goes on the wall down to the tub. Will there be a shower arm installed with this tub?

Green board does not go down to floor it goes over flange of tub and marble goes over flange and green board. If you bought a fiberglass tub screw it and level it with flat head drywall screws or drywall nails to the studs. Also if it is fiberglass put a bed of stuctolite plaster under it so there is no give in tub when you get in it.

And it will not be greenboard if you have a shower arm.