From the Super Mario Wiki
Moon Coins are types of Star Coins that only appear in World Star of New Super Mario Bros. 2. They have moons that resemble a 3-Up Moon imprinted on them, rather than stars. As such, they cannot be used as Star Coins, meaning that they cannot be used to remove Star Coin Signs; collecting Moon Coins is only required to obtain a star icon on the player's profile.
Moon Coins also appear in Mario Golf: World Tour where they can be obtained by the player by accomplishing challenges.
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