Cut away peel and pith from grapefruits and oranges. Cut each fruit in half lengthwise, then slice crosswise into ¼-inch- thick pieces. Transfer fruit to bowl and toss with sugar and ½ teaspoon salt. Set aside for 15 minutes.
Melt butter in 8-inch skillet over medium heat. Add pecans and ½ teaspoon salt and cook, stirring often, until lightly browned and fragrant, 2 to 4 minutes. Transfer pecans to paper towel–lined plate and set aside.
Drain fruit in colander set over bowl, reserving 2 tablespoons juices. Transfer fruit to platter, arrange in even layer, and drizzle with oil. Whisk shallot, mustard, and reserved juices together in medium bowl. Add watercress, ½ cup figs, and ¼ cup pecans and toss to combine.
Arrange watercress mixture over fruit, leaving 1-inch border around edges. Sprinkle with remaining ½ cup figs and remaining ¼ cup pecans. Season with salt and pepper to taste. Serve immediately.