We had a similar problem. I discovered it when I was taking down the previous owner's wallpaper. It led to tearing out all the drywall above the (fiberglass) enclosure, and then behind the toilet & vanity too.

We replaced it with greenboard.

Greenboard is no longer an accepted material for a wet area. It's about a "half a spit" better than drywall as it has a waxy coat on it. Just put up new drywall and paint on a coat of Redgard before painting it.....