Combine the quinoa, 2 cups water and 1/2 teaspoon salt in a saucepan and bring to a boil over high heat. Reduce the heat to low and simmer, uncovered, until the water is absorbed, 12 to 14 minutes.
Meanwhile, grate 1 teaspoon orange zest, then juice all 3 oranges and the tangerines into a bowl. Sprinkle the sugar in a skillet and cook over medium-high heat until dark amber, about 5 minutes. Remove from the heat and whisk in the vinegar, citrus juices and zest, and the coriander. Return to medium heat and boil until thick, about 8 minutes. Remove from the heat and whisk in the butter; keep warm.
Season the scallops on one side with salt and pepper. Heat the oil in a large cast-iron skillet over medium heat. Add the scallops seasoned-side down and sear until golden, 4 to 5 minutes. Flip and cook until golden on the other side, 2 to 3 more minutes.
Fluff the quinoa with a fork and divide among plates. Top with the scallops, drizzle with the citrus sauce and sprinkle with the parsley.