Preheat the oven to 350°F (175°C). Spray a 13x9-inch casserole dish with nonstick spray and set aside.
Thaw the frozen corn and drain well (microwave 30–60 seconds or run under cold water until thawed, then drain).
In a medium bowl, whisk together the Greek yogurt, skim milk, and salsa until smooth.
Add the shredded chicken, drained corn, diced red bell pepper, and half of the shredded cheddar cheese to the yogurt mixture; stir until evenly combined. Reserve the remaining cheese.
Arrange 4 of the 6-inch corn tortillas in a single layer on the bottom of the prepared casserole dish, overlapping as needed to cover the base.
Spread 1/3 of the chicken mixture evenly over the first tortilla layer.
Place 4 more tortillas in a single layer over the filling.
Spread a second 1/3 of the chicken mixture evenly over the second tortilla layer.
Place the last 4 tortillas in a single layer over that filling.
Spread the remaining 1/3 of the chicken mixture evenly over the top tortillas, then sprinkle the reserved shredded cheddar evenly over the top.
Cover the casserole with aluminum foil and bake for 30 minutes, until the filling is hot and bubbly.
Remove the foil and bake an additional 10 minutes, until the cheese is lightly browned.
Let the casserole rest for 10 minutes before slicing and serving.