I have found that you have to get stains out before you wash them because it seems that the washing sets them. My granddaughter 2 years old wrote with a marker all several blocks of a Wedding Ring Quilt I had taken years to make for my DD. She brough it to me. I put palmolive dish detergent (white is best but green works) and kept blotting it then I ran water through it. Soaked it with Palmolive & washed it. A little stain was left so I dipped it (not soaked) it in a strong bleach solution and the rest came out.
I would try the bleach on this but just try dipping the back in the bleach water and then scrub with your fists and see how it works. Maybe a little piece at a time.