Rinse the glutinous rice under cold water until the water runs mostly clear, then drain well and add it to the slow cooker.
Pour in 1 cup of the coconut milk along with the water, sugar, and salt. Stir to combine.
Cover and cook on low for 3 hours, stirring once halfway through to keep the rice from sticking to the bottom.
Stir in the remaining coconut milk and cook for 30 more minutes until the pudding reaches a thick, creamy consistency.
Turn off the heat and let the pudding sit for 10 minutes. Divide into bowls and top with diced mango and toasted coconut flakes before serving.