Preheat oven to 350º. Stir together cream cheese, mayo, sour cream, garlic powder, and paprika until combined.
Fold in spinach, cooked bacon, Parmesan, and 3/4 cup mozzarella. Transfer spinach mixture to baking dish and sprinkle with remaining 1/4 cup mozzarella. (If mixture is very high in baking dish, place a baking sheet below dish to catch spillage.)
Bake until golden and bubbly, 25 to 30 minutes.
Make crostini: Drizzle bread with olive oil and season with salt. Toast until golden, 10 minutes. Rub with a garlic clove while still warm.