If it makes you feel any more comfortable about using the name ‘Etheria’, it is also the name of the planet that She-Ra lives on in the ‘Masters of the Universe’ line. My assumption is that if that has been copyrighted, then it is owned by Mattel.
A quick check of the US govt Trademark Database shows that there are no live Trademarks for Etheria. Which is good for you. Trademarks are generally harder to get around than copyrights. Copyrights tend to be context based. If there is no troll king and evil fairies in your Etheria.. or a girl who transforms when she holds aloft a magic sword then you’re pretty much set. Its just a name afterall and there are only so many letters that you can work with.
I’m not a lawyer so if you’re really worried you might want to check with one. But this is how I understand the laws to work.