In a small bowl, whisk together olive oil, balsamic vinegar, minced shallot, garlic, salt, and pepper. Set aside.
Cut tomatoes into wedges or chunks. If desired, remove seeds to reduce excess liquid.
Slice cucumber in half lengthwise, then cut crosswise into half-moons about 1/2-inch thick.
In a large bowl, combine tomatoes, cucumber, and mozzarella.
Pour the dressing over the salad and toss gently to coat evenly.
Let the salad rest for 5 minutes to allow flavors to meld.
Just before serving, fold in the basil chiffonade.
Sprinkle with flaky sea salt if desired and serve immediately.