Are you occaissionally scraping the lawn clippings from underneath the deck? I would think it(mold) would need some type of "agent" like that along with moisture for mold to develop. Caw....caw....caw(if you are confused, let me explain that those three words represent the sounds of djdicetn "eating crow"). After buying my Gravely Zero Turn and mowing six times or so with it I never saw ANY grass clipping/dirt/etc. "buildup" underneath the deck(only what appeared to be a "green stain"), like the buildup that I had with both of the previous stamped deck lawn tractors I owned. So I posted several times here that I thought my Gravely was "immune" to that build-up.......WRONG!!! Now that I've mowed a few more times(have a little over 12 hours on it now) I am finding a "thin layer" of build-up under the deck(hard to tell, but seems to be a combination of grass and dirt). By no means is it "huge clumps" like the stamped deck lawn tractors had and it is VERY easy to remove with a putty knife in 10 minutes or so(I spent 30-45 minutes on the old lawn tractors washing all the clumps of build-up off the underside of their decks....or else take a chance throwing a belt). But I'll bet, even though the build-up on the Gravely would not affect the blade travel/drive belts, that if I don't scrape that thin layer off every once and a while that moisture on and underneath that layer of grass/dirt could very well lead to mold, rust and other not-so-nice side effects. So, now that I have admitted I was wrong about "no build-up" on my better-than-perfect Gravely......could this be related to your mold problem??????
P.S.
Prior to 2010, "eating crow" was known widely here in Tennessee as meaning that you were admitting you were wrong about something you had prviously said.......but in 2010 Tennessee legislators(for lack of better things to do) passed the infamous "Tennessee Road Kill Law" that stated that if you "accidentally" killed wildlife....it was legally yours for consumption(like hitting a deer with your vehicle). Now "eating crow" is also widely known as a reference to a nice home-cooked Tennessee meal:0)