Using a fork, combine the salmon, scallion, lemon zest, 3 tablespoons yogurt, 1 teaspoon mustard, 1 teaspoon horseradish, salt and black pepper in a medium bowl. Add the wheat germ and celery and season with salt and pepper. Mix well.
Coat a medium nonstick skillet with cooking spray and place over medium heat for about 1 minute. Mix the capers, lemon juice, and oil in a medium bowl.
Add the arugula and toss to coat. Transfer to a serving plate, forming a bed of the arugula mixture. Set aside. cook 2 minutes per side, then flip and cook 1 minute more, or until heated through and lightly browned.
Place on top of the reserved arugula mixture.
Combine the remaining one half teaspoon yogurt, 1 teaspoon mustard and 1 teaspoon horseradish in a small bowl. Spoon on top of the salmon cakes.