Would it just fall right through whatever it's sat on or would it instantly expand into something less dense?
If somehow you could keep it confined, it would drop through whatever it was put on, through the entire Earth, and pop up at the other side. I suppose it would keep oscillating and eventually settle in the core.
Ok, on that note... what would it expand as? I mean, would it be a physical substance, like particles of something like a gas or a plasma?
I assume it'd be quite hot (putting it mildly!)
If kmm's guesstimate of 5e26 joules is accurate, we're well beyond "extinguishing multicellular life" and in the realm of eliminating Earth entirely, which requires a minimum of 5.97e24 joules: https://en.wikipedia.org/wiki/Gravitational_binding_energy 1% efficiency of transfer to the Earth is probably attainable.
The energy density contained within a bit of neutron star matter makes a big explosion, and that will likely dominate over the more exotic processes.
Not sure what that ejecta would look like...but if I had to guess, it would be release a shitload of energy as it came out from under compression, and would eventually condense into normal matter, much of it comprised of the sort of heavy elements not easily created via stellar nucleosynthesis.
Imagine a cloud of incandescent radioactive gold dust weighing as much as multiple earths, and you've got the idea.