brake cleaner will get it off....GUARANTEED.
ppl will flame for the stuff being harsh...but it will clean it off.
I spray some on a rag / water...comes off like butter.
did it on my USDM bumper for about 5 months...
then got a tune / JDM bumper...no more yellow stains.

 

If nothing else works you could always wetsand and buff......... 2000 grit and a buffer always does the trick.

 

grease lightning............. and rewax.....


**** works super good and is not as harsh as others... I've been using it for 2 years,,