I'm just guessing, not knowing anything about the topology of your cooker. But my guess is that you are getting moisture dripping onto your food, carrying residue from the top of your smoker which generally includes some very light ash which has drifted in from the fire.
On my most recent horizontal offset build, that's why I used a round tank, so the moisture would run down the sides past the food, not drip onto the food.
Also, you may have some kind of turbulence carrying light wood ash in from the firebox. Do you clean out ashes after every cook?
I myself personally was taught to scrape out the heavy and then pressure wash the inside of my cooker after every single cook. My cooker smells sweet and I heavily doubt I'm missing any flavor by cleaning off big gobs of grease. Rancid grease doesn't smell good and it for sure doesn't taste good. The pressure washing leaves a very fine film and doesn't remove seasoning. My racks are shiny black and so is the entire inside of my cooker. No mold after winter, either.
My cooker has a 2" drain pipe with ball valve, and another 2" cleanout pipe with ball valve. Those 2" ball valves are very pricey if you buy new. So scrounge.