A: All cards in the discard pile are considered to be “not hidden.” You can look through them at any time. So when Composting is in play, you are considered to be able to look at any and all cards you might possibly draw. Since the card you play with Mystery Play is supposed to be a mystery, Andy rules that, no, you may not pull your Mystery Play from the bottom of the discard pile.

Note that you may be seeing this FAQ because your deck has a version of this card with a different name. There are so many analogues for Mystery Play that you should simply check the Fluxx Comparison Chart. Highlight the row for the Rule: Mystery Play, and scroll across to the deck you have to find the card this question refers to.