Mix first 3 ingredients; stir in 2 Tbsp. Alfredo sauce. Spread onto bottom of 9-inch pie plate. Top with spaghetti sauce.
Spoon remaining Alfredo sauce into resealable plastic bag. Cut small corner off bottom; drizzle in spiral design over spaghetti sauce. Pull toothpick through sauce to resemble spider web.
Bake 20 min. Add 1 olive half for the spider's body. Cut remaining olive half for the spider's legs. Serve with crackers.