Cut the cucumber lengthwise into quarters, then cut into ¼ inch pieces. Place in a large bowl.
Remove the tails from the shrimp and chop them into bite-sized pieces. Add to the cucumber and toss to combine.
Add the green onion, dill, garlic, mayonnaise, lemon juice, and mustard and stir until everything is well coated.
Season to taste with salt and pepper.
