To prepare relish, combine first 4 ingredients in a medium glass bowl. Microwave at high 30 seconds or until sugar dissolves. Cool. Stir in onion, parsley, and squash. Cover and refrigerate 1 to 4 hours.
To prepare salmon, sprinkle fillets with 1/4 teaspoon salt. Place fillets, skin sides down, on grill rack coated with cooking spray; cover and cook for 9 minutes. Combine mustard and honey in a small bowl; brush over fillets. Cover and cook 2 minutes or until fish flakes easily when tested with a fork. Serve with relish.