Rinse sweet rice until water runs clear. Soak rice in warm water, 2 to 4 hours. Drain.
Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Line the steamer with cheese cloth and spoon rice into cheese cloth; steam until thoroughly cooked, about 20 minutes. Remove steamer from heat and transfer rice to a bowl.
Mix coconut milk, sugar, and salt together in a saucepan; bring to a simmer. Pour 3/4 of the coconut milk mixture over rice; cover bowl and let sit for 5 minutes.
Pour remaining coconut milk mixture over rice and top with mango slices.
