From the Super Mario Wiki
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Hidden Coins (also known as Dash Coins) are special types of coins that have been seen in the New Super Mario Bros. games, Yoshi's New Island and Super Mario Run. Hidden Coins are somewhat rare, and only appear in a few levels.
Hidden Coins first appear as an outline of a Coin; if Mario or another character passes through the outline, it will then turn into a regular coin after a moment.