For ground flock, take some coffee grounds after you’ve used them, put the on a glass or ceramic saucer, put it in the microwave and heat it until it’s bone dry, then put it in an airtight container like a small old butter tub.
This stuff looks like rough dirt, is the right color for it, is easy to glue to a base and you can easily paint it with acrylic based paint.
Plus it’s basically free. You probably either use coffee or know someone who does, so acquiring an amount of used coffee grounds for free should be easy. Millions of people throw it in the trash daily.
Years ago when I was doing this with gw figs before I could no longer tolerate their business practices I used this on like 3 gazillion ifgures and never had a problem. The few gw figs I still own aren’t molding afaict.