Many succulents are toxic if ingested. Others have mild medicinal properties (like aloe) where if ingested in large amounts it can also do damage. I like to refer to this list
of safe succulents when I'm designing something for my reptiles. It should apply to hedgehogs as well but I can't promise anything. Your best bet is either a succulent that is completely safe with every species it's been tested on, a fake one, or elevating it in a way that your hedgehog doesn't have access to it.