For the salsa: Peel the mango and cut into 1/4 inch diced pieces.
Put mango pieces in a small bowl with the remaining salsa ingredients; stir to combine.
Cover bowl with saran wrap and refrigerate until ready to serve.
For the marinade: In a small bowl, whisk together the soy sauce, sweet sake, vegetable oil, light brown sugar, fresh ginger, and minced garlic; set aside.
Place the mahi mahi fillets in a large zip-lock plastic bag.
Pour marinade into bag; press the air out of the bag and seal tightly.
Turn/shake the bag to coat fillets with marinade.
Refrigerate for 20-30 minutes.
Take fillets out of bag and throw away marinade.
Brush or spray both sides of fillets with vegetable oil.
Grill over high heat until fish is opaque throughout, 8 to 10 minutes, turning once halfway through grilling time.