View attachment 334991
Darn, my iPad has it, too. The picture is after I looked and cleaned it up a bit. The plastic strip isn't totally smooth and has some pits in it. It looked almost moldy or mildewy but was probably oxidized aluminum. Almost all of it wiped off the iPad except for a spot that feels scratched into the metal. I just put a strip of clear tape along that side of the iPad to prevent further scratching.
My cover comes off a few times a week both intentionally and accidentally. I've never thought about wiping or dusting off the parts of the iPad and cover that touch, so I probably trapped dirt and other grit in between them. I don't recall ever wiping anything other than the screen as long as I've had the iPad.