From the Super Mario Wiki
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Flages are enemies that appeared in Kid Icarus: Uprising. They resemble a gelatinous mass with one eye and blades on it hands. They will often move around invisible, the only indication of their presence being a slight green fog. However, they will reveal themselves to attack.
Flages also appear in Super Smash Bros. for Nintendo 3DS as Smash Run enemies. Like in the original game, they will move around invisible and unable to be attacked by the player, with the only indication of them being present is their shadow. They attack by swiping the blades on their hands multiple times at the player, though they reveal themselves to attack, allowing the player to attack them.
Super Smash Bros. for Nintendo 3DS Trophy information