Summarize this article:

Isopropyl alcohol is a solvent that will dislodge any color from stains. This includes the green pigment left behind by grass. To wet the stain, take a sponge or cotton swab and generously dab with alcohol.  Rubbing alcohol, also known as isopropyl alcohol, works on removing grass stains because it dissolves the green pigment left behind in a grass stain. If you’re working on a delicate fabric, try a 50:50 solution of water and alcohol. Note that adding water means it may take longer to dry. Allow the stain to complete air dry before moving forward. The alcohol will evaporate out of the stain and most of the pigment should be dislodged. After the stain is dry, rinse it with cool water. Using cool water prevents the stain from setting. Use of hot water, or heat at all, will set the stain and make it more difficult to remove. Apply a small amount of detergent to the stain. Massage for at least five minutes, but the longer the better. Once you’re satisfied with your massaging, rinse the stain with cold water until the water runs clear. Allow the garment to air dry. Once it is dry, check to see if the stain is gone. If not, repeat the process. If the stain has been removed, you can launder the item as normal.

Summary:
Wet the stain with isopropyl alcohol. Air dry and rinse. Apply liquid detergent. Check the stain.