How do I know the mouth guard I use for grinding is not toxic? I feel bits of it in my throat on bad nights.

Sometimes. Sometimes we need Government Oversight, although less often than our politicians think. The materials used in Occlusal Orthotics, night guards custom made for you by your Dentist, are safe & non-toxic. If you have concerns, call the dentist that made the appliance for you.
If it. Was made at a dental office more likely the materials used are ada fda approved. If that is the case you should not have any concerns about toxicity.