In a small bowl, whisk together the pesto, olive oil, white wine vinegar, and honey until well combined and emulsified. Set the dressing aside. While that rests, slice the nectarines into 1/2-inch thick wedges, slice the cucumber into 1/4-inch rounds, halve the cherry tomatoes, and tear the fresh basil leaves into bite-sized pieces. I like to taste the dressing at this point and adjust the acidity or sweetness if needed—a touch more vinegar or honey can really balance the stone fruit perfectly.
In a large bowl, gently combine the sliced nectarines, cucumber rounds, halved tomatoes, and mozzarella pearls. Pour the dressing from Step 1 over the top and gently toss to coat everything evenly, being careful not to break up the delicate fruit and cheese. Fold in the torn basil leaves just before plating so they stay vibrant and don't bruise.
Transfer the salad to a serving platter or individual plates, arranging the nectarines and mozzarella in an attractive pattern if desired. Scatter the toasted pine nuts over the top for a nice textural contrast. Just before serving, drizzle the balsamic glaze over the entire salad and finish with flaky sea salt. I save the balsamic glaze for the very end because the acidity can cause the delicate basil and nectarines to wilt if they sit too long.