I checked everything on Nihilism and jellyfish, and found nothing. What I did find, however, is that there is a song/band/album/something like that called Nihilist Jellyfish :P
Also, I looked up several definitions of Nihilism and got this:
the rejection of all religious and moral principles, often in the belief that life is meaningless.
extreme skepticism maintaining that nothing in the world has a real existence.
Nihilism: That nothing matters, which may make sense because Nihilego has no real sense of value in anything; nothing really seems to matter for it...
That is pretty much it though. It could also mean both: the Japanese is a combination of void, ego, and being similar; All of the other names use ego and void, hollow, etc, which is like nihilism i. e. not caring/mattering.
TL;DR: It probably is Ego and Nihilist because other languages use ego and void/other words considered synonymous with Nihilism.