Diagnosis
That's baked-on grease and carbon, and it's actually doing part of its job—the reverse flow plate is a seasoning layer. But too much gunk can affect heat reflection and airflow.
How to Fix It
After your cook, while the smoker is still warm (not scorching hot), scrape the plate with a sturdy putty knife or a dedicated grill scraper. For a deeper clean, once it's cooled, you can take it out (it's heavy, get help). Lay it on some concrete or a tarp, hit it with a degreaser (like Simple Green or a citrus-based cleaner), let it soak, then scrub with a stiff brush and rinse.
If That Didn't Work
For really stubborn deposits, some people carefully use a propane torch to burn off the crud, then scrape. This is messy and smoky, so do it outside away from anything flammable.
Heads Up
Don't use oven cleaner or harsh chemicals unless you plan to re-season the entire smoker thoroughly afterwards, as they can strip all seasoning and leave a residue. A little carbon buildup is fine and helps with even heating.