The Mystery Mold item is obtained from Black Fungi in Kingdom Hearts. It is a puzzling substance left behind by a Black Fungus when it is defeated with a Critical Hit. It has no purpose except bragging rights, though it can be sold for 9999 Munny.
A Black Fungus has a spherical, pitch black head with two glowing yellow, beady eyes. It wears a round, purple cap on its head that sports two black, angular spirals on its front. It also seems to wear a navy blue smock that completely covers its legs and thin, fingerless arms. Its Heartless emblem is present on the front of this smock.
The Black Fungus's name references its dark coloration, as well as Fungi, a biological kingdom that contains organisms such as mushrooms and truffles.
Unlike the other Mushroom Heartless, it is very aggressive. It can become invulnerable for a short time to defend itself and release poisonous gases, making it very hard to defeat. If you finish them off with a critical hit at the end of a combo, they may either drop a Mystery Goo (100%), or a Mystery Mold (10%). It is possible for them to drop both, and upon being finished with a Critical Hit, they drop around 20 to 100 Munny.
If you're having trouble finding Mystery Mold when you find a Black Fungus, use Trinity Limit. If you don't have Trinity Limit yet, use Gravity, as it is usually a one-hit knockout. Since the Black Fungus drop Mystery Mold after critical hits, make sure to remove all Combo Pluses before you fight them to shorten the length of your combo.
In Kingdom Hearts: Chain of Memories and Kingdom Hearts Re:Chain of Memories, the cards it uses are numbered 7, so be sure to break them accordingly. Defeating Black Fungi usually gives you a Calm Bounty card, so exhausting Black Room cards is a rather easy way to stock up on Calm Bounty cards.