Awful problem to have.
Have seen this problem before (unfortunately with black mould), but maybe not to the same extent as shown in the photographs. Almost makes me think that the leather has NOT been cleaned with a proper leather cleaner and this has really accelerated the whole mould growth process by either removing the protective coating on the leather and/or providing a ready source of food.
Mould loves to feed on organic matter (in this case, leather) in conditions where moisture is present and there is warmth to help things along.
Please note that mould spores, depending on type, can be hazardous to health if breathed in, so best to wear a mask when cleaning up the mess.
1. Ventilate car in direct sunlight
2. Use a stiff brush (nail brush or old tooth brush) to loosen mould, and then vacuum up with a Hoover.
3. Mix distilled white vinegar with tap water (maybe a ratio of 4 vinegar to 1 tap water, or even neat vinegar).
4. Spray mixture over all areas affected by the mould, and then wipe in with a cloth. Leave to soak for about 30 minutes to kill off any spores.
5. Allow to dry out naturally or suck up any remaining vinegar/water with a wet/dry vacuum cleaner.
6. Repeat process until all mould is eradicated.
7. Assess car to understand how moisture is getting in (especially under carpets and around door/window seals). Fix if necessary. If nothing is obvious, a desiccant sack on the dash is useful for absorbing normal moisture in the atmosphere.
Hope this helps.