That white, powdery bloom or dark spotting is the obvious sign. But it's rarely the only one. Stiffness that wasn't there before. A surface that's gone dry, dull, or brittle where it used to be soft. Leather doesn't fail all at once, and mould is often just the most visible symptom of a piece that's been drying out, losing its natural oils, or sitting in the wrong conditions for longer than you realised.

Leather is porous and alive in its own way. It needs conditioning the way skin needs moisture, and without it, damage compounds. Dry, untreated leather cracks more easily and gives mould an easier foothold once humidity's involved; mould, in turn, feeds on the material itself, not just the surface. By the time you can see either one, it's usually been developing for a while in the fibres underneath. The stiffness stays. The smell comes back. A faint stain reappears a few weeks later.