That's a plastic trim that can be removed. Probably is missing some clips. If you have some plastic tool, you could try to gently remove it and you'll see how it's mounted and see if there are any clips missing. I've done some work on my car and had to remove this trim. There is a lot of dust and/or fine dirt accumulating under. A good opportunity for you to clean that area. Did you replace the back window recently? The trim must come off when they replace the back window and maybe the trim was not reinstalled properly.