In your slow cooker, combine water, quinoa, sweet potatoes, black beans, salsa, bell pepper, onion, green onion, cilantro, and taco seasoning. Stir everything together.
Cover and cook on high for 2-3 hours or on low for 4-6 hours, until the quinoa is cooked and the liquid has been absorbed.
Remove the lid and give the casserole a good stir. Taste and adjust seasoning with salt and pepper as needed.
Mix in 1 cup of shredded cheese, then sprinkle the remaining cup on top.
Put the lid back on and cook for an additional 10-15 minutes until the cheese is melted and gooey.
Serve immediately with your favorite toppings, like Greek yogurt, avocado, extra cilantro, or tortilla chips for scooping.