Preheat a grill to medium-high. In a small saucepan, add 2 cups water, cover and bring to a boil over high heat. Place the couscous in a shallow, heatproof bowl and pour the boiling water on top, reserving the pot. Stir in the olive oil, then cover tightly and let stand for 5 minutes.
Fluff the couscous with a fork, then stir in the mustard seeds, cucumber, parsley and cider vinegar; season with salt and pepper.
Add the balsamic vinegar and thyme to the reserved saucepan, cover and bring to a boil on the grill, about 5 minutes. Uncover and simmer until the vinegar is syrupy and reduced to 1 tablespoon, 5 to 7 minutes. Carefully remove from the heat and discard the thyme.
Increase the grill heat to high. Season the salmon with salt and pepper. Arrange skin side down on an oiled grate, cover and grill until the skin is crisp, about 3 minutes. Flip and grill until just cooked through, 1 to 2 minutes. Drizzle the balsamic reduction over the salmon and serve with the couscous