Heat a grill or grill pan to medium-high.
Rub the mahimahi with 1 teaspoon of the oil and season with 1/4 teaspoon salt and 1/8 teaspoon pepper. Grill until cooked through and opaque, about 5 minutes per side, depending on the thickness.
Meanwhile, cut away the peel and white pith of the grapefruit with a knife and slice the fruit into 1/4-inch rounds.
In a small bowl, combine the remaining oil, lime juice, honey, 1/2 teaspoon salt, 1/8 teaspoon pepper, and scallions.
Divide the watercress, avocado, and grapefruit among individual plates. Place the mahimahi on top and drizzle with the lime dressing.