What is reptibark made of? I know that I saw some awhile back that said it was made of "fir bark" and fir is a pine type tree, so that might not be good. But if it is that ground-up cypress stuff, you're fine. that's what I use, and I have never seen mold on it.
I would be kind of concerned about the moistness of the substrate in your tank; if it can support the mold, maybe it could be a tad too moist for the snake? If it's a shed cycle, that's OK, but during regular time I let my cages get fairly dry. Of course they always have a water bowl, so there's no extreme drying or anyhthing.