Preheat grill for medium heat and lightly oil the grate.
Whisk lime juice, olive oil, garlic, soy sauce, mustard, salt, and pepper together in a bowl; brush over mahi mahi to coat.
Bake on the preheated grill for 5 minutes, flip, brush again with the lime juice mixture, and continue grilling until the fish flakes easily with a fork, about 5 minutes more.
