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

Meanwhile, bring rice, water, chicken bouillon, and 1 tablespoon butter to a boil in a saucepan over high heat. Reduce heat to medium-low, cover, and simmer until liquid has been absorbed, about 20 minutes. Pour in coconut milk and 2 tablespoons sugar; stir. Simmer, uncovered, until coconut milk mostly absorbed.

Set an oven rack in the oven's center and preheat the oven's broiler.

Remove mahi mahi from 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.

Meanwhile, melt 1 1/2 tablespoons sugar and 1 1/2 teaspoons butter in a skillet over medium-high heat until begins to bubble; stir in mango. Cook and stir until mango is tender, about 5 minutes.

Place 1 mahi mahi fillet over 1 scoop coconut rice; top with mango salsa.