You may already have experienced it: in springtime, when unpacking the whips after a long winter, looking forward to do some cracks, you suddenly see an ugly, grey stuff coating the entire whip, sometimes up to the handle. Obviously, the whole whip mildewed during the last months. Soon the poor whip gets (mis)treated with bleach or aggressive cleaners - unnecessarily, as the described condition is just a deposit of waxy components of the leather dressing. This is caused by changes in temperature and/or humidity and also by excessive use of leather dressing. With a magnifying glass, one may see the deposits, appearing like sugar crusts.
To make sure it is no mold, simply warm up an area. The deposit melts and disappears (at least for a while). Mold would stay unchanged. In this image, an area was just warmed by thumb:
As a remedy, warm the affected parts carefully(!) with a hairdryer at low temperature and rub the surface with a lint-free cloth. This may need to be repeated from time to time. The deposit is annoying, but not dangerous.