Molecule vs Crystal

In the case of ice, the hydrogen and oxygen atoms are actually sharing electrons. They are bonded "covalently". Each oxygen is bonded to two particular hydrogens, and so you can divide the atoms into separate groups: this oxygen is bonded to these hydrogens, and that one to those, and so on. That is not the case in NaCl, where really the different atoms are all attracted to each other electromagnetically. No sodium atoms are actually bonded to any particular single chlorine atom, so it makes no sense to speak of one of the molecules in a crystal.

The difference between a liquid and a fluid is not well defined. Highly viscous fluids (like pitch in the famous pitch drop experiment) are solid at normal time scales but flow on very long time scales. Glasses are by definition amorphous, so show no long range spatial order (like a crystal); as a result they are not in their ground state and must have a finite but very long relaxation time. Amorphous solids have the highest degree of symmetry (isotropy); the transition from amorphous glass to crystalline (SiO2 say) involves a symmetry breaking.

So, first there is the issue of solid vs liquid (some finite but potentially very large viscosity). Then there is the issue of amorphous versus crystalline solids, a transition from continuous to discrete symmetry. Sodium Chloride FYI has a melting point of 800 C.

An interesting tangent is non-newtonian "fluids" like silly putty, which flow under gravity, but when struck with a hammer will crack. Also in the news recently scientists at Corning measure flow in an enormous block of Gorilla glass. But the thickening of mediaeval stain glass at the bottom is NOT the result of flow.