In a large bowl, combine the cooked shrimp, mayonnaise, Dijon mustard, lemon juice, celery, red onion, garlic powder, salt, and pepper. Mix well until all ingredients are evenly coated.
Cover the bowl with plastic wrap and refrigerate the shrimp salad for at least 30 minutes to allow the flavors to meld.
While the salad is chilling, slice the cucumbers into 1/2-inch thick rounds and arrange them on a serving platter.
Once the shrimp salad is ready, spoon a generous amount onto each cucumber slice.
Garnish with fresh dill or parsley if desired, and serve immediately.