Black Fungus are Evil Little Fellows, emitting Poisonous gasses, turning Invincible but they are nice in a way after you defeat them.

Black Fungus Information

Although White Mushrooms are Nice and Friendly, the Black Fungus is Evil. They Can be some of the most Trickiest Enemies in the Game, as they emit a poisonous gas that causes lots of damage. When they Turn a light grey color, they become Invincible for a short period of time, and they can still release the poisonous gas while in this form. Back off for a while and wait until they return to normal form.

Mystery Mold

On a Rare Occasion, the Black Fungus will drop a mystery mold. You must have all your characters equipped with lucky strike in order to better increase your chance of getting a Mystery Mold. If you kill a Black Fungus with a Critical Hit, your chances of getting a Mystery Mold is 50%. You can sell the Mystery Mold for 9,999 Munny! That’s a lot, eh? Anyways, the Mystery Mold will not be dropped during the Pegasus Cup when you fight some Mystery Mold.
Black Fungus Appearance Locations

Agrabah: Bazaar
Halloween Town: Moonlight Hill
Olympus Coliseum: Pegasus Cup, 4th Seed
Olympus Coliseum: Hades Cup, 36th Seed
Hollow Bastion: Cell
End of the World: Linked Worlds