Chlorine dioxide is used for bleaching of wood pulp and for the disinfection (called chlorination) of municipal drinking water, treatment of water in oil & gas applications, disinfection in the food industry, microbiological control in cooling towers, and textile bleaching.

It may be the case that this is a legitimate MMS kind of thing- the FDA would definitely confiscate that, but it may not be as harmful as it sounds. Not sure I'd wear a pouch of it round my neck though.