I use crusty salt, when I accidently forget to tighten the lid down on my bucket it gets crusty some times, its fine IMO but he should have made sure it was completly dissolved though. without fully dissolving there are now still chemical reations balancing out in your aquarium. the danger would lie in one of the "flakes" landing on or getting injested by something, either chemicaly burning it or making it sick. or in unstable water parmeters as the stuff disolves.