I found some mould on my biltong this morning
I googled around and found that this is caused by insufficient air flow (eg: too humid in the biltong box). The good news is that it doesn't mean the meat is spoiled - you just have to wipe it down with some vinegar and rehang it.
I have also added a little 4" desk fan to the biltong box, as clearly the 60w bulb wasn't causing enough temperature differential to move the air around. I am going to add some more air holes too.
Anyway, just thought I'd share this lesson learned with y'all.