Acid is an element found in the Pikmin game series, and found in many, many Fanon games. Acid hazards take the form of either a corrosive green gas or a bright green liquid. All but Green Pikmin will instantly die if they touch acid, and Captains cannot walk in it either unless they have a certain upgrade.
Some enemies also utilize acid as a weapon, for instance, the Jungle Bulborb leaves behind puddles of acid when angered. Green Pikmin generally should be used against acid-wielding enemies.
In Pikmin: Lair Of The Umbrax Dweevil, Mushroom Pikmin are resistant to acid. Pikmin run around in little circles melting if they touch acid, and then die, unless a Captain whistles at them or a Mushroom Pikmin helps out.