Stir coconut milk, 2 tablespoons cilantro, 2 tablespoons green onion, lime juice, ginger, garlic, and fish sauce together in a saucepan; bring to a boil.
Brush the mahi mahi fillets with enough sauce to coat. Cook the remaining sauce at a boil until slightly thickened, 3 to 4 minutes.
Cook the mahi mahi in the grill pan until the fish flakes easily with a fork, about 7 minutes per side.
Transfer to a serving platter, top with the sauce, and garnish with 2 tablespoons cilantr and 2 tablespoons green onion to serve.
