The good thing about the sunlight method is you do not need to accurately diagnose the problem.
If the substance on the leather is the oils & salts coming out of the leather, the warm dry air will help. If it is mold, the UV light and dry air will help. If it’s a little of both, the sunlight will help.
After the leather is clean and dry, then you have the decision of how to best proceed. Everyone has their own favorite method. Some folks like beeswax, some folks like mink oil, some folks like Ballistol, and the list goes on.
All leather is different and the methods to tan leather are different, but I’ve yet to see the sunlight method fail as a starting point.