Preheat oven to 375 degrees F (190 degrees C). Spray 2 baking sheets with cooking spray.
Lay a sheet of waxed paper onto a work surface. Unroll crescent dough and separate into 16 individual triangles. Cut each triangle in half lengthwise creating 32 isosceles triangles. Arrange triangles on the waxed paper.
Sprinkle a pinch of mozzarella cheese in a thin line parallel to the shortest edge of each triangle, ensuring the cheese doesn't touch the edges. Top the mozzarella cheese with Parmesan cheese and parsley flakes.
Roll the short end of each triangle halfway toward the top point, creating a rim, and sprinkle garlic powder over dough. Place a mushroom half on half of the triangles, midway between the rim and the top point, and a pepperoni half in the same place on the remaining triangles. Place 1 or 2 olive slivers against the rim to form a "ribbon" or "belt" on each "hat". Place the "hats" on the prepared baking sheets.
Bake in the preheated oven until golden brown, 5 to 7 minutes.