Stir in scallion whites, beans with liquid, garlic powder, chili powder, cumin, ¼ tsp salt, and pepper. Bring to a boil over medium-high heat. Once boiling, reduce heat to medium. Simmer, uncovered, for 5 minutes, then mash beans until mostly smooth. Continue simmering until mixture has thickened, about 3 minutes more. Turn off heat. Transfer bean mixture to a bowl. Wipe out pan.