The mist card is a mysterious spirit who is really good at eroding and rotting things. It's mist could erode anything but it isn't acid and will not hurt living things. It can, however hurt people by eroding things and it did it in episode 14 when Sakura went to see a play. It eroded everything on the stage, which almost hurt two actors. Sakura sealed it but how? I don't really know.

The mist card creates a yellowish mist that engulfs anything and rots and corrodes it when it is summoned by the captor.