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.
My father in law built it. It's basically a frame of 42x19mm pine (1 1/2" x 3/4") with 3mm (1/8") plywood sides. There are dowels with little stainless steel hooks (he used MIG wire - you could use paperclips). The false floor (with little holes in it) has a light bulb under it (the sheet stops the juices landing on the bulb). There are air holes above the dowels on the sides, and also at the bottom. All the holes are covered by flywire, and there's a door (open for the photo, obviously) the seals up the front.
The little fan is sitting on the false floor pointing up (this photo was taken after 24 hours - biltong's been in there for about 5 days now)
That is really cool. I just have have strings running from window frame to the other side of the room. Tie them up tight and leave the ceiling fan on. Does get distracting when I'm working to have 3lbs of meat hanging over my head.
So my first batch is ready. It's too salty, but still edible (actually, it's very tasty - you just need a glass of water afterwards!). Lesson learned - need to wash the salt off better in the vinegar bath.
And when I put the next batch in, I took another photo of the cabinet that shows the arrangement with the fan and false floor a little better.
I did a bit of a write up on my blog for anyone that isn't already familiar with how to make biltong.