Prepare fish marinade: Whisk together olive oil, soy sauce, lemon juice, garlic, and ginger in a bowl. Season with red pepper flakes, black pepper, and salt. Stir in green onion. Add mahi mahi and toss to coat evenly. Cover the bowl with plastic wrap and marinate in the refrigerator for 1 hour.

Meanwhile, make coconut rice: Bring rice, water, chicken bouillon, and butter to a boil in a saucepan over high heat. Reduce heat to medium-low, cover, and simmer until the liquid has been absorbed, about 20 minutes. Pour in coconut milk and sugar. Stir and simmer uncovered until the rice has absorbed most of the coconut milk.

Preheat the oven's broiler and set the oven rack in the middle of the oven.

Remove mahi mahi from the marinade and shake off excess. Discard remaining marinade. Place fish in a large baking dish in a single layer. Broil in the preheated oven until the fish flakes easily with a fork, 10 to 15 minutes. If fish browns too quickly, cover the baking dish with aluminum foil.

While fish cooks, make mango salsa: Melt sugar and butter in a skillet over medium-high heat. When mixture begins to bubble, stir in mango cubes. Cook and stir until mango is tender, about 5 minutes.

Serve by placing one mahi mahi fillet over a scoop of hot coconut rice, and top with mango salsa.