First time home owner here. I stuck these vent deflectors a month ago on one of our vents and I removed it today for some other reason. I noticed these stains that looked like water stains but I think it's actually something that is growing - maybe unfortunately mold or something. I need help in understanding this situation and possibilities of this happening. Please share your experience or knowledge related to this issue if you can.
Thank you!
EDIT: Mystery solved - this seems to be the result of some chemical spray during cleaning. Not concerning.
Call a tech and get your heater serviced. Yes you're supposed to get those looked at annually. When they're doing your heater, ask them what they think.