if you are going to install that surround tub enclosure and you install it correctly and take great care with the seams or corners, then you could get away with Green Board.
But first, I would tear out all the wet and moisture laden areas, Direct a fan into the area to completely dry out the area. You do not want any moisture, mold, or algea growing from this.
Spray down the area with bleach to kill the stuff then do the drying and open ventillation.
Put it back with Green Board.
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