In medium saucepan, heat oil over medium heat. Add scallions and celery and cook, stirring occasionally, until softened, about 4 minutes. Add tomatillos, garlic, oregano, coriander and chili powder and cook, stirring occasionally, 4 minutes. Stir in 1 1/2 cups water, cover, increase heat to high and bring to a boil. Add Squash and salt and return to a boil. Reduce heat to medium-low, cover partially and simmer 5 minutes. Stir in bell pepper and continue simmering, partially covered, until Squash is almost tender, 3 to 5 minutes.