Originally Posted by Detailed Image
Most likely it's some dried up polish, sealant or wax. The longer it's been on the surface the more challenging it will be to remove. If it was very fresh almost any light degreaser could remove it but as the sun bakes it in it gets harder. I typically use the Poorboy's World Trim Restorer
a few times and it's usually pretty successful. Hopefully this will do the trick for you as well.
Greg @ DI
Thanks for the input Greg. I will add that to the list