I used to use Galco Royal Guards a long time back. I used to use two of them a year, usually rotating one out for a new one each year. In the summer, they were both constantly wet through from sweat and never would dry out. There was usually discoloration, but I never remember seeing mold on them. Not saying it wasnt there though.
If this is an issue for you, Id highly suggest you check out a good kydex holster. It will eliminate 99.9% of any kind of moisture issues you have with leather, and the holsters are usually about half what a good leather costs.
This was the last Royal Guard I had. Its about a month old in the pic. The duct tape on the back was there to try and slow the sweat from getting through. It was a lost cause......
This is the Blade Tech that replaced it. Its got over 10 years of daily use on it. I still have it today, and its still just as useable as the day I bought it about 20 years ago.....
“The enemy is anybody who's going to get you killed, no matter which side he is on.” ― Joseph Heller, Catch-22
Beware of associating with "Cheap" Squirrels. They tend to be very narrow minded, hypocritical, and will gag you if you question or disagree.