Unfortunately, it's a common mistake. Greenboard was actually removed from the IRC book maybe even a few years ago???? Bottom line is it's faced with paper. Paper and water = food for mold. I keep posting this picture here but it's one of the best examples of why greenboard is obsolete.
Densarmor is sold at Menards and Lowes...as far as I know, Homey Depot doesn't care it. Like I said, it is the exact same procedure as installing drywall. I use it exclusively in any bathroom remodel I do. Taking it 1 step further, I also use fiberglass tape for the seams as using paper tape is kind of silly if you take the effort of not using paper on the drywall.
I use GP's drywall mud made for Densarmor too. Regular drywall mud has some organics in it that are mold food.