To clean grass stains on washable fabrics, creation a solution of 1 part rubbing alcohol to 16 parts water. Apply the solution to the stain. Rub it in and then let it sit for a couple of minutes and then rinse with cool water. If the stain is still there, you can repeat this several times. You can also try using hydrogen peroxide instead of rubbing alcohol, but this has a greater chance of affecting the color of the fabric so test it out on a small area first.