Thank you so much for going so detailed! That is so much good information and brainstorming. I washed it with soap - it still stays, I doesn’t go away or even reduce a bit.
Spray - possible. My cleaning people come over once every two weeks and clean windows and blinds as well. This vent happens to be right above a window.
I also run a humidifier in the room at night. No other room has this much gunk on it. Only my room does. So that could be one more contributing factor.
The substance is not oily. It is not powdery either. It seems like almost etched into the plastic - this may support your spraying theory: someone cleaned something nearby with sprays that could damage acrylic surfaces.
That was a good investigation! Awesome person you are!