Hi Jimble, is it a painted bumper or non painted?
Assuming its painted!!
Try and get as much as you can off during the wash process, I would suggest getting it off by hand polishing with a medium to course compound like farecla g3 formula scratch paste or liquid or similar,
don't need to go mad with pressure or anything like that but the abrasives will cut through it with ease, you will need to refine it after though to jewel the finish.