[lugerholsterrepair]

Tom, First thing you have to do is determine if it IS mold. It might not be. It probably is but rub your finger over a good portion of it and see if your finger is greasy.
I have often seen old leather holsters leach out a white surface stuff that looks like mold but isn't. It is oils or fats surfacing that had been put on the holster years ago. I have found no good way to get rid of this stuff except to rub it off with a soft cloth. Assuming it will re surface.
If it is genuinely determined to be mold, I have had good luck with brushing it off with a soft brush since it should be dry and powdery and freezing the holster. This should go a long way towards killing the spores. You may have to do it several times as mold spores are encapsulated and may not be affected by the freezing process untill open.
The last time this conversation surfaced one of the members suggested a commercial mold treatment for use on boats. I forget exactly what it is. I will look thru my old Email and see if I can find it.
One other way I have thought of it to deprive the mold of oxygen for a time. I am going to try stuffing the holster gently with paper and placing it in a food saver bag and vacuuming out most of the air. If the vacuum dosen't smoosh the holster it would be worth a try.
Maybe some of the other members have some ideas?
Jerry Burney