Using a fork, combine the salmon, scallion, lemon peel, 3 Tablespoons plain Oikos Greek Yogurt, 1 teaspoon mustard, and 1 teaspoon of the horseradish in a medium bowl. Add the wheat germ and celery and season with salt and freshly ground black pepper. Mix well.
Coat a medium nonstick skillet with canola oil cooking spray and place over medium heat 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.
Divide the salmon mixture into 2 round patties, each 1/2'' thick. Reduce the heat to low and add the patties to the skillet. 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 ½ teaspoon plain Oikos Greek Yogurt, 1 teaspoon mustard, and 1 teaspoon horseradish in a small bowl. Spoon on top of the salmon cakes.