noticed this odd marking on the slide and not sure when it showed up haha. (weird squiggly thing behind the barrel).
That's just from the ejected brass "kissing" the slide as it's flung out. I have several pistols that get marks like these (usually ones with matte or bead-blasted finishes on the slide, such as my Walther P99 or Colt XSE).
A little Hoppes #9 and a good scrub with a stiff nylon-bristled brush will take those marks right off.