In a large bowl, combine the sweet rice flour and matcha powder; whisk to blend evenly.
Gradually add about 1/4 cup of water and stir until the mixture starts to come together.
Add more water little by little (total 3/4 to 1 cup) until the mixture forms a dough that is soft but not runny; if too wet, sprinkle in a bit more rice flour.
Lightly dust a work surface with rice flour, transfer the dough onto it, and knead briefly until smooth.
Shape the dough into a log and divide it into 8 equal pieces (you may make 12 smaller pieces if preferred). Roll each piece into a ball.
Flatten a dough ball in your floured palm to about 1/2 inch thick, place about 1 tablespoon of red bean paste in the center, and wrap the dough around it, sealing the seams.
Repeat with the remaining dough and filling, placing finished mochi on a lightly floured tray.
Heat and lightly grease a grill, grill pan, or skillet over medium heat.
Grill the mochi, 4–5 minutes per side, until they develop light browning and slightly crisp edges.
Transfer cooked mochi to a plate and serve hot.