Mushroom Pikmin, or Puffmin, are the result of allowing Pikmin to be exposed to the spore cloud attack from a Puffstool. Pikmin affected by the cloud turn purple, grow mushrooms on their heads, and turn against Olimar and other Pikmin.
Mushroom Pikmin can be cured by being shaken off Olimar, by defeating the Puffstool, or by waiting for the effect to wear off. Attacking a puffmin with an unaffected Pikmin also has a chance to revert it back to its original Pikmin color.