Combine first 3 ingredients in a bowl. Cover; microwave at HIGH 3 minutes or until tender. Stir after 1 1/2 minutes. Drain.
Heat oil in a large nonstick skillet coated with cooking spray over medium-high heat. Add corn, bell pepper, and garlic; sauté 5 minutes or until tender. Stir in potato mixture, milk, and salt. Add cheeses, stirring until cheeses melt.
Warm tortillas according to package directions. Divide corn mixture evenly down center of each tortilla; roll up. Top each serving with 2 tablespoons onions and 2 tablespoons salsa. Serve immediately.