Although Neutronium is used in SF a lot, I got a different take on it from a video of a researcher who put his iPhone in a neutron beam to see what happens (but that's another story).
He pointed out that a marble sized ball of neutronium would not only need a SF "force field" to keep it under pressure, but also be in time stasis because it would give off as much energy as some rediculous number of atomic weapons per second.
A time stasis field would block anything inside it from view (in Larry Nivin's stories they are perfect mirrors) and can't be effectivly transparent because the stopped time inside would stop the light from coming or going.
If it were contained and stabilized in some manner that did not cloak it, what might it look like? Bulk objects reflect light selectively to give color, due to the electrons blending into a net of sorts that have many ways to stash energy so "take" light of any frequency. A pure element gas would show spectral lines, coming or going as the case may be, and ignores light of different frequencies because there are only specific energy levels present. So, for example, it would be perfectly transparent except for a narrow bit of yellow taken out. Hey, sounds like air, nice and clear to our range of frequencies.
So are the electrons in our sample a mesh like a mineral grain, finiky like a gas, or liquidy like a metal? Uh... what electrons? Actually any charged particle will do. Nope, none of any kind, just neutrons.
So, it would be perfectly transparent. Not considering gravity lensing, and the effects of whatever is stabilizing it. Or maybe not... if it's really "like a neutron star" you would have other stuff in it at equilibrium, plus a crust of other materials covers it so you can't see it anyway so let's suppose pure contained neutrons somehow. But, in a story setting that would give you licence to make it clear, black, white, or mirror, "depending".
Actually, neutrons have a concentric separation of charges, so from the outside the concentric charges cancel out, but might that give some degrees of freedom for light to interact with? If the skin's charges can somehow blend together and work like a mineral grain, I think it would not react to anything less than hard x-rays, so still invisible to our eyes.
Effects of a very small electric dipole moment (and I do mean very) beyond the Standard Model would be correspondingly small, so don't expect anything visible from unknown effects yet to be discovered.
Does magnetism affect light, if very strong? That's a question for another topic. If you could get all the neutrons to line up their fields...