Fiberglass and acrylic tub and shower enclosures come with a flanged edge that attaches directly to the wall studs and the proper way to hide this flange is to install drywall over it.
Rotten wall behind bathroom enclosure.
Drop those pieces into a bucket of hot water to separate the gypsum backing from the tiles.
If you find that the wood is rotted you will need to replace it.
If any of the wood studs appear wet and moldy you can clean them with a water bleach solution.
Make sure the bottom side edge of the drywall sheet extends down over the thin lip of the tub s mounting flange.
Install an air barrier behind showers and tubs installed on exterior walls.
Leave some of the old drywall exposed where it will meet the new backerboard.
Then pull the tile and small squares of drywall out together.
Install insulation without misalignments compressions gaps or voids in all exterior wall cavities behind tubs and showers.
Any insulation that is wet or moldy will also need to be replaced.
Allow the wood and wall cavity to completely dry.
Leave a 1 8 inch gap between the bottom.