I would take a bleach/water solution and spray the inside of the center section, let in sit for 15 minutes or so, then scrub with a plastic brush. Rinse well and light a hot fire and let it burn out.
I keep mine in the garage with the vents all closed and the cover on it here in Florida. Also the next day after every cook I empty the used charcoal, save it, and take a 4" paint brush and sweep all the ash dust off of the charcoal grate and the inside of the bottom bowl. Charcoal ash draws moisture in the WSM and a kettle. Leave it in there too long, you get rust, mold, ash sweep froze up and an environment for mold. Trust me here in SW Florida mold grows like crazy, steady work pressure cleaning pool decks, sidewalks, drive ways etc. for mold.
2 years, I never have seen any mold in mine.
I heard the Weber covers are not rain proof, to fix that, put a black garbage bag over it, then put the Weber cover over that to prevent rain leaking in