From the Super Mario Wiki
Sneaky Spirits are enemies that appeared in a mini-game from the Rhythm Heaven series. Sneaky Spirits appear in Super Smash Bros. for Nintendo 3DS as Smash Run enemies. They attack by popping out of the ground and going back to it. After using attack multiple times, they quickly jump out of the ground and try to hit the player. They move quickly and can even disappear, but they will drop star stat boosts, which can raise every stat. One event in Smash Run spawns many Sneaky Spirits and only Sneaky Spirits for a limited amount of time.
Super Smash Bros. for Nintendo 3DS Trophy information