Grass is soluble in acetone, so this is the way that it is removed. Pour some acetone into a glass (don’t use plastic), and then scrunch up the garment and immerse the part of the garment containing the grass into the acetone. You should see it begin to dissolve immediately. Gently lift the garment out, then reinsert and so on. Repeated dipping is of the garment in the acetone will eventually see the grass fully removed from the garment.
Since this mostly occurs in cricket whites the colour of the garment is not an issue, but if you are using a coloured garment test an inconspicuous part with the acetone for colour fastness.
Isopropanol may also be used, but it is not as aggressive a solvent as the acetone and it will take a little longer.