you typically cannot get mystery goo from White Mushrooms. you've gotta find rare truffles (the blue mushrooms). you can typically find these in Monstro's throat (you'll have to have defeated monstro to access this, or at least the parasite cage for the first time.)
cast aero and strike the rare truffle, causing it to soar up in the air. position yourself underneath it and let it bounce off you. remember to recast aero frequently. DON'T let aero run out or let truffle-guy touch the ground, or he will become most upset and leave. i'd suggest loading sora up with ethers or mega-ethers before this, since casting aero repeatedly will consume a lot of MP. i'm not sure if aero will be sufficient--you might have to already have the upgrade to Aerora or Aeroga.
if there are no rare truffles in the throat, try going back to the mouth, into chamber one, then back out into the mouth and into the throat. you have to go at least two locations away to make things appear when you want them to.
the other spot where you can find a Rare Truffle is in the Hercules Cup at Olympus Coliseum.