Heat oil in a large pot over medium-high heat. Cook and stir chicken strips until browned and cooked through, 5 to 8 minutes. Remove with a slotted spoon to a bowl.
Cook red, green, and yellow bell peppers, and sliced in the pot until peppers are tender and onions are translucent, 5 to 7 minutes. Transfer chicken back to the pot. Add chili powder, cumin, garlic powder, salt, and pepper. Stir to coat chicken and vegetables evenly with seasonings.
Pour milk into the pot and stir in pasta. Bring to a simmer; cook until sauce thickens and pasta is tender yet firm to the bite, 10 to 12 minutes. Remove from heat.
Stir Cheddar cheese into the pasta mixture until cheese is melted. Serve in bowls topped with sliced green onions.