So I did an experiment last night before I went to bed - I put a folded paper towel in the tank, held tight by the glass top just like the stocking, and I also folded the top of the stocking back into the water (no Matrix in it, just the nylon). Both were pink this morning.
It dawned on me that it must be dyes leaching into the water from the substrate. That tank was set up before I ever knew about dirted tanks, and it's a mix of natural color gravel and CaribSea FloraMax. My guess is it's the FloraMax, which isn't quite the same dark color it used to be. Guess I'll be hanging a HOB with Purigen on that tank when I get home from work tonight! But what a shock it was to walk into the room and see that last night - lol.