It's the dirt from the car. As I noted, Optimum says it's the effectiveness of the ONR at breaking down the dirt particles so small that they penetrate deep into the media, where they are difficult to remove. They claim that because the particles are so small, they pose no risk to the paint. I tried to find a thread about this on the OPT forum, but when Chris Thomas (forum rep/moderator) left he deleted all his posts, destroying a lot of content there.
One time I had a piece of tar stuck in wash media like I have pictured, and pulled the glob off, then sprayed some OPC on that spot before laundering. After laundering, that spot that I sprayed looked nice and clean, without the gray staining, so I have been playing around with using OPC as a pre-wash, but I haven't gotten all the way there (to solving the problem) yet.
When the wash media is wet, that staining looks black, rather than the gray when it is dry. Even if the staining came out easily in the laundry, it still is a problem for me when trying to discern if I've gotten a panel clean, if the media turns black on the first pass and doesn't rinse out. I'll point out as I have in the past, that the media that I have pictured ("The Bone" is on the right) both use "non-split" microfiber, which makes them different (I guess) from a regular MF towel.