Super Dave - yes that is the correct piece. From what I can see it's just a plastic trim strip. There is a space between the two pieces of diamond plate there. I'm not so concerned about the plastic trim strip it's the diamond plate on the bottom side that is bowed out a bit leaving a gap for water and junk to get in.
Unfortunately I think Bedlam may be correct. To properly do it that whole piece may need to come off. It just doesn't seem like there is any adhesive on the back side. But I'm not sure how to get the diamond plate to lay flat again.
Here is the response from Northwood Mfg that leads me to believe Bedlam is on the right track - "if you didn't want to replace the diamondplate, i would find a flat molding and install it there . and be sure to seal it with geocell. staple the diamondplate on both sides to make it lay flat. seal it and mold it. then seal the molding with geocell"
I don't think I have the know how or tools to replace that whole sheet of diamond plate. I used to weld aluminum but that was many moons ago and don't have the equipment anymore