There definitely weren't any chemicals spilled on it, even water. I'm extra cautious about this stuff. However, I actually asked this question in another "forum" (a facebook group, sorry not sorry), and the response I got was that this is actually normal, and due to the difference in material between the two terminals. The positive terminal is soldered onto the PCB and thus most likely nickel coated copper, whereas the ground lead is likely iron based with some sort of easily-oxidizing coating.
To support this, I took a screenshot from EEVBlog's video on the DHO800. The terminals are also different looking there. I suppose one outstanding question could be why is the difference so strong in my case but not in the video? Although there's many contributing factors there, chiefly different lighting conditions and the black front panel of the 924S. I was shining a strong flashlight at the terminals in my photo to highlight the difference; compare with the photo I took just now without any direct light on the scope.
Still not sure if that's conclusive, but I've been using the scope for a while today and at least it doesn't seem to be progressing. I had no contact spray on hand to see if it helps, but I ordered some and will try it when it gets here.