Careful pressure washing. You will need to use a low to moderate pressure washer to remove the stain.

A couple of pointers. First you will need to be certain to use the correct tip. You will need one which, when held 8 – 10 inches from the surface is the same width as your deck boards. Second, you will need to begin at one end of each deck board and continue to the other without stopping. Stopping and starting in the deck boards will leave marks and these will show through any new stains you apply.