Rear Under Panel Fastener

[mks]

I'm attempting to remove a bunch of road debris (stones and dirt) that has collected in/on the rear under paneling. However, I don't recognize one of the fasteners used to secure it.

Is this a type of push pin or screw? In any case, I've no idea how to remove it. Your expertise is much apprecaited, thanks.

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[D/\rK•650]

Hello I was wondering what you figured for this fastener? I believe it's the plastic one with the straight piece that inserts into this hole. I realize this is very old post it's just popped up on my post list so I checked it out.