What is on my dryer vent cover?
The dark mark is a substance that melted from the heat. The unidentified mark appears to be a plastic material (at least in my opinion). This could be anything from a candy or cigarette wrapper to ziplock baggie.
Most likely it was in some ones pocket and went unnoticed after washing. If there is still a film remaining it would be wise to scrape it off so it doesn't transfer to any brilliant whites clothing in future dryer usage.