Build a low fire in a charcoal grill or heat a gas grill to medium-low. Remove salmon from marinade and set aside on a plate to come to room-temperature. Meanwhile, pour marinade into a 2-qt. saucepan, and bring to a boil over medium-high heat; cook, stirring occasionally, until reduced by half, about 15 minutes. Place salmon on grill, and cook, turning once and basting with boiled marinade regularly, until just cooked through, about 9 minutes. Transfer to serving plates, and drizzle with remaining marinade; garnish with scallions and sesame seeds.