Preheat outdoor grill to medium heat and oil the grates using a brush or paper towel.
In a small bowl, mix the chile powder, salt, and pepper together. Coat the yellowtail fillets with a light layer of olive oil and sprinkle with the chile mixture. Place each fillet flesh side down on a hot grill and cook for about 3 minutes. Flip fillets over and cook for an additional 3 minutes. Remove from grill and top with Mango Salsa.
To make mango salsa, combine all ingredients together in a medium bowl and stir well. Keep refrigerated until ready to use.