Grass stains can usually be removed if they are treated as soon as possible after they occur.
Pretreat or soak the stain in a product containing enzymes. This can be a presoak product, a detergent booster or a liquid laundry detergent. Wash the garment as usual. A chlorine or oxygen bleach (whichever is safe for the fabric) added to the wash will help remove any remaining stains.