In a medium pot, bring the water to a boil. Add tapioca pearls and cook over medium heat for 12 to 15 minutes until mostly translucent, stirring often.
Drain the pearls in a fine sieve and rinse under cold water to stop the cooking. Set aside.
In the same pot, warm coconut milk with sugar over low heat until sugar dissolves. Stir in cooked tapioca pearls.
Chill the mixture in the fridge for at least 1 hour for best texture.
Before serving, fold in most of the diced mango. Spoon into bowls and top with the remaining mango and mint leaves.