HEAT oven to 350ºF.
MIX cream cheese, 2 Tbsp. milk and garlic powder until blended. Combine noodles, tuna, peas, mushrooms and remaining milk in large bowl. Add cream cheese mixture; mix lightly. Spoon into 1-1/2-qt. casserole sprayed with cooking spray; cover.
BAKE 20 min. Meanwhile, mix cracker crumbs, Parmesan and butter until blended.
STIR noodle mixture; top with crumbs. Bake, uncovered, 5 min. or until crumb topping is golden brown.