Hi,
You can get a special product to remove mould from leather, however I have found that a quick once over with some leather cleaner should remove it. Generally its just mould growth on the leathers finish. If it re-occurs after removing it then I would use a mould remover, like the ones you would use on carpets just to kill the bacteria.
And if the mould is a black stain then use a leather stain remover to remove it.
What you have to remember with leather, is that the majority of problems are on the finish of the leather. So normally any problem like a stain, mould, dirt can eb quite easily removed using the correct products.