That is one of the things I am working on(testing) In nature, the hogs on my site, nest in silty sand. No plant material or rich soils of any kind.
In captivity, I am testing a sand/coco fiber mix and the exact same sand they used in nature. I can't say which is better yet, but neither molded, which is your question. Also a sphagnum/sand mix does not mold either. Good luck and post your results please. Best wishes