Using a mandolin or very sharp knife, thinly slice zucchini lengthwise. Place in a colander; sprinkle with salt. Let stand 30 minutes. Rinse and drain; pat dry with paper towels.
Preheat oven to 400 degrees F. Place focaccia on a baking sheet.Prink generously with a fork. Bake 5 to 10 minutes until lightly toasted and heated through. Remove from oven; set aside.
Meanwhile, in a 12-inch skillet heat 2 Tbsp. of the olive oil over medium-high heat. Working in batches, slightly brown zucchini slices on both sides, about 5 minutes per batch. Transfer zucchini to a bowl. Sprinkle with crushed red pepper, if desired. Remove skillet from heat. Add remaining olive oil and balsamic vinegar to the same skillet. Spoon balsamic mixture over the focaccia.
Place focaccia on a serving tray. Top with zucchini, prosciutto, and arugula (arranging each in lengthwise rows, if desired). To serve, cut crosswise into slices.