Hi David, you might consider either of these methods:
1. Apply Citrus Gel and rinse with hot water.
2. Apply Stain Pro and allow to stand for 2-5 minutes.
3. Rinse thoroughly with water and blot until dry.
(From Prochem stain removal guide).
Alternatively:
Grass Stain contains: Tannin,acids,oils and chlorophyll.
1. Apply P.O.G (if available) to remove chlorophyll.
2. Apply Liquid Enzyme Spotter detergent and blot.
3. Rinse with water and blot.
4. Apply Alkaline Spotter solution and blot.
5. Apply Acid Solution Spotter and blot.
6. Rinse with water, blot until dry.
7. Remaining green mark may need and application of Dye Gone or Spray & Go to remove it.
(From Chemspec spotting guide).
Remember other spotters are available too.
Hope this helps.
Regards
Jim