Cook the pasta in well-salted water until al dente. Drain and rinse under cold water to stop cooking.
While pasta cools, slice pepperoni (halve if large), chop cucumber into bite-sized pieces, halve cherry tomatoes, and thinly slice red onion.
In a large bowl, combine cooled pasta, pepperoni, veggies, olives, and mozzarella. Mix lightly.
Add Italian dressing gradually, tossing gently to coat evenly. Taste and adjust salt and pepper as needed.
Cover and refrigerate for at least 30-60 minutes. Before serving, toss again and add a splash of dressing if dry.