Peat Moss can have all kinds of stuff in it. It is potting soil which can include parasites or weird bacteria or whatever. Probably generally safe for most animals BUT if this garter is sensitive to something maybe not so safe.

If I were concerned and wanted the natural earth type look I would consider eco-earth or something similar for this enclosure instead. I am a newbie and not an expert, I like the look of eco-earth and bought some but am afraid of humidity levels being too high.