Moss like that is typically a chemically preserved kind. It's not marketed towards reptiles, so I don't know if any of the products they use to preserve can be hazardous.
If you do use it for decor, I would rinse it out very well and let it fully dry again before anything else.
If you use it to decorate the outside of the hide, that may be best as you can also keep it from becoming too moist. My main concern would be the coloring agent staining your snake's scales or the possibility of the moss being knocked into water if left loose and leeching chemicals or dye into it.
I was using some fabric ivy decorations for enrichment with my snakes but had to really let them soak for a bit before using them. A lot of excess color rinsed out with a bit of dish soap that I'm glad did not make it into my enclosures and onto my snakes
@_@;