Here is what you can try:
Dry out the jacket in warm room temperature until completely dry.
Remove surface mould with a damp cloth. If leather is dry, mould will wipe off.
Allow the jacket to dry a second time.
Clean dried jacket with a leather cleaner or a very mild shampoo such as baby shampoo.
Keep the jacket properly stored to in dry area to prevent the mould from returning.
Never use a detergent.
Do a spot test and allow it to dry before cleaning the entire jacket.
If the home method does not work, contact a "professional" dry cleaner.