Anybody have any idea what this stuff inside the cap of my pen is?

I was riding with a Wing Sung 3009 today (a pretty nice, super cheap piston filler BTW) when I noticed this gunk on the inside of the cap. I'm pretty sure that I filled the pen up with Noodler's Air Corps Blue/Black a few months ago and then let the pen sit. It wrote pretty much right away.

But I noticed this brownish / tan / black residue on the inside of a cap, at the very bottom. I'm not sure if it was there before and I just didn't notice, or if it is new. It almost looks like burned plastic? Or maybe it's mold? But it seems like a weird place for mold to develop since there's no ink there. Has anybody seen anything like this before? I'm probably going to throw the pen away anyway (it's about $3.50) but was curious 🧐.