ToS's OST (and presumably other things with English/Engrish written on it) called it Tethe Alla too.
It? What is "it"? What's your source, here, Cyllya? I've never heard of anything like that before....Selene?
Tales of Symphonia. I think that bit was in some bookshelf text somewhere. Tower of Mana, probably, but I don't think I have a save file where I can check the Tower of Mana's books.
EDIT: Ah, I do have a good save. The relevant book, "Tethe'alla Observation Record," is in the upper left corner of the Tower of Mana entry room. It says....
Tethe'alla is Sylvarant's moon, which orbits this planet once every forty-two days. Prior to the Ancient War, Tethe'alla was known as Selene. The Church of Martel advocated the name, Tethe'alla, after the Ancient War. The name comes from a legend that the people of Tethe'alla migrated to the moon.
And that's why all the Sylvarant characters get confused when someone mentions Tethe'alla not refering to the moon.