Preheat oven to 350℉. Drain the green chiles. Cut the cream cheese and Velveeta into cubes.
In a medium pot, melt the butter on medium. Stir in the flour, salt, and pepper. Cook 1–2 min., until flour is golden, stirring constantly. Gradually whisk in the milk and heat to a simmer. Cook 2–3 min., until thickened, stirring constantly. Stir in cream cheese, green chiles, and Velveeta until melted. Drain any excess liquid from corn and fold into cheese sauce. Season with salt and pepper.
Transfer mixture to an oven safe 1½- to 2-quart casserole dish. Top with the Cheddar and bake 10–15 min., until cheese is melted. Thinly slice the green onion and use as garnish.