Is this mold... or just dirt/debris?

We just moved into our house in August and when I went down to the basement to replace the humidifier water panel I found the cover on the floor. I'm assuming the inspector didn't put it back on correctly during the inspection. Since the system is in a little nook in the basement I never noticed the cover fell off (hindsight being 20/20, it explains the drastic temperature difference in the basement compared to the rest of the house). Anyways, I noticed what appears to be dirt or debris (hopefully not mold), in the duct near where the panel fell off. Would anybody who's reading this minding giving me their input as to what it could be? Thanks in advance!