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.
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