Frozen Coins are objects appearing infrequently throughout the Super Mario series. In all games they appear in, they are coins encased in solid ice that can be melted by fire to be collected.
Super Mario Bros. 3 / Super Mario Advance 4: Super Mario Bros. 3[edit]
Frozen Coins first appear in Super Mario Bros. 3 at the end of World 6-7, and then appear throughout World 6-10. The player can melt the ice and collect the coins within by using their Fire form's fireballs.
Frozen Coins were added in the version 2.0 update for Super Mario Maker 2, exhibiting the same behavior as in previous games. They can now be melted by any fire-based course element with the exception of lava, and the melting process is noticeably faster as well. Exclusively in the New Super Mario Bros. U theme, the spells used by the Koopalings can melt Frozen Coins.
Play Nintendo: "These coins are surrounded by a block of ice and can only be released if melted by fireballs or other fire elements, including a certain angry sun."[1]