In a medium bowl, add soy sauce, sugar, vinegar, garlic powder, and ginger; whisk to combine. Add seitan; stir to coat evenly. Set aside for 10 minutes.
In a medium skillet over medium heat, add seitan with marinade. Add tapioca starch; stir to combine. Stir continuously as the sauce thickens, making sure to scrape the bottom of the pan. Cook for 5-7 minutes, until the sauce is gooey.
Add bell pepper and mango. Cook for 8-10 minutes, or until peppers are tender. Add a bit of water if needed to keep the sauce from burning.
Serve the stir fry over jasmine rice and top with green onion.