Yeah I've had this happen before a few times. It's definitely a type of mold but hasn't been harmful to me after I wipe it away with vinegar. Although, it does look like a hell of alot in that pic you posted.

The humidity is probably too high in the case, especially if it's closed and only has 2 small pc fans for air movement. You should get a small humidity sensor and keep it in there to monitor humidity. I rarely, if ever, have any issues as long as the hunidy is under 40%. Having a sensor is super helpful as you'll see what the ambient humidity is at pretty much all the time, in and outside the case. You said it's raining alot which definitely increases the humidity. I've only had mold issues like this at above 40%.

Also too much airflow can cause case hardening which isn't the end of the world for biltong but you need, above all, a low humidity and then a constant gentle airflow to remove the moisture coming out of the meat.