Combine 6 cups water and rice in a large bowl and soak 8 hours to overnight.

Drain rice and rinse with fresh water. Combine 2 cups water and rice in a saucepan and bring to a boil. Reduce heat to medium-low, cover, and simmer until rice is tender and water has been absorbed, 15 to 20 minutes. Transfer cooked rice to a bowl.

Combine coconut milk, sugar, and salt in a small saucepan. Heat until just boiling; pour mixture over cooked rice. Let stand 30 minutes. Serve warm with sliced mango.