Cut the ahi into one-inch cubes. Add the fish to a medium size bowl. Pour the limejuice over the fish and mix well so each piece is coated in limejuice. Cover with plastic wrap and allow to rest in fridge for 20 minutes.
After the fish has rested, pour off about half of the excess limejuice. Add the cherry tomatoes, sweet onion, carrot, cucumber and coconut milk and mix well. Add salt and pepper to taste. Garnish with sliced green onions. Serve cold.