Activated charcoal wouldn't be on the fresh air side of the filter, with the same texture and appearance of mold. I've seen black mold, cleaned it off of flat surfaces, and didn't have any confidence at all of getting this completely out of, or off of, the filter. Just a guesstimate, but if it was mold, then the filter performed it's designed task, and kept whatever it was out of the passages buried deep in the dash. Not a bad outcome.