Black fungus in kingdom hearts won't give mystery goo how is mystery goo obtained?

You can get myster goo by hitting a rare truffle into the air 10 times, the more you dubble the number the better chance you can get the myster goo and also better stuff, hit it in the air 10, 20, 30, 40, or 50 times to better the chance of getting it, use Aero then stand under it to hit it into the air.

or

You can use the white mushrooms to get it, it's more rare to get the myster goo with them but you still can.

To get the myster goo from them, you have to hit them three times in a row with the smae type of magic to get them to drop the myster goo. The gestures they make will tell you what type of spell they want you to use on them:

Shivering - Fire

"Bring it" gesture - Ice

Sun over head - Thunder

Trip and Fall - Cure

Spinning around - Aero

Frozen in place - Stop

Floating in air - Gravity

You can also kill the black fungus to get the Myster Goo but that is very rare to get it from them, if you want to get it from them, use the magic stop on them then attack them, it works every time so that they can't attack you back. =)

Hopefully this helps you!!! =)