Back in 2014 when I was doing research on my 2015 SGS and what exhaust I planned on getting, I remember reading a post on HDforums about Rinehart end caps and how they turn sort of purplish after a while. Google rinehart end caps, and you'll probably find it. If I recall, it said to go out and ride your bike, heat up the mufflers then use a rubber mallet after fasteners are out. They "supposedly" come off easier when hot.
You might also try a product called PB Blaster (spray for loosening rusted marine fasteners). Then, tap it a few times with rubber mallet after the blaster sits for awhile. Also a strap wrench to get 'em wobbling back and forth.