Oh I don't think those would be safe at all. To me, those seem like a major problem, especially if they're a dollar store one. You don't know if they're gonna break apart quickly. The moss my float around and irritate their gills. And I have seen a video of vet removing a stone almost the size of an axolotls head, it was stuck in their mouth. Really sad so I wouldn't do it unless it's at least twice the size of your axolotl's head.