Mystery Coin (XIPM), like Fools Gold is a single mint Chia CAT token. What makes it radically different is it's max supply of only 6187 XIPM. Mystery Coin is only distributed inside the Fools Gold community as a token of gratitude to those who participate in beta test activities and is currently being used to beta test the upcoming faucet distribution of Fools Gold (XIP). If you want to get a hold of some of the very limited supply head on over to the Fools Gold Discord.
Also like Fools Gold it is completely useless, without monetary value, and will only be given out for free for being an active member of the community (at least by its creators).