Not sure if mold is a problem, but I think that weeds growing between the stones after 3-4 years can be.
I asked your questions to a landscaper a few years ago and was told that they like installing stone. Why? They get paid to install it, then paid again to remove it a few years later. Just what I was told.
I like to take the master gardeners advice.
All About Mulch - Gardening Solutions - University of Florida, Institute of Food and Agricultural Sciences