Rinse tapioca pearls under cold water.
In a medium saucepan, bring 2 cups of water to a boil. Add tapioca and reduce to a simmer. Cook for 15 minutes, stirring often.
Turn off the heat and let tapioca sit for 10 more minutes, covered.
Drain any excess water and rinse tapioca under cold water to remove excess starch.
Transfer tapioca to a bowl and stir in sugar and coconut milk.
Refrigerate for 1 to 2 hours to chill.
Divide into serving bowls and top with diced mango. Garnish with mint if desired.