in the cave and you have to get the code to get in the room the has it and the code is on the top of the cave
The green "key ingredient" on Shark Tooth Island is part of the potion to put the giant shark (Booga) to sleep. It is down in the underground ruins, along with another ingredient, the creature bone. Open the underground door and go left to find the bone and then the key ingredient. (You already have the third ingredient, which is coconut milk.) (see related question)
Mask? What mask? There is no mask that you have to find in Shark Tooth Island, Poptropica.
Go on the roof and get the translation key. Go in the temple and use that to go inside. Deep in the temple you should find the secret ingredient. Feed that to the shark.
You have to go inside the temple and get to the other side. after each obstical you find an ingredient. you also need the coconut milk.
Climb back up past the worm statue, go left to find the chamber with the final (key) ingredient,
You have the medicine man mix a sleeping potion, using coconut milk, a creature bone, and a key ingredient. The bone and green ingredient are down in the underground ruins. Once you have it mixed, you fire it from the shark feeding Coconut Cannon at Booga bay. (see related questions)
In the Temple
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First, you go into the temple. then, you just find the last ingredient! But beware of the bats!
I'm assuming you mean the key ingredient to make the medicine on Shark Tooth Island. The key ingredient isn't anything specific; it's a sack. You find it in a cave.
When you have sedated the shark, you can swim out to the island in Booga Bay to bring him back.