Preheat oven to 400 degrees F (200 degrees C). Spray muffin cups with cooking spray.
Combine teff flour, brown rice flour, arrowroot starch, brown sugar, baking powder, cinnamon, and salt in a bowl.
Mix water, vegetable oil, and eggs together in a separate bowl; whisk into flour mixture. Stir in bananas and walnuts. Pour batter into muffin cups to 3/4-full.
Bake in preheated oven until tops spring back when lightly pressed, 20 to 25 minutes.
