Zalia is not included in
Smadar Shir Sidi's
The Complete Book of Hebrew Baby Names. It is therefore rare or has been created since the book was published in 1989.
Perhaps one guess would be that Zalia is someone's "Anglicization" of Tzlila, a name which is also spelled Zlila in
Israel and which is the feminine form of Tzlil, which means "tone, sound" in Hebrew. But that is just a guess, based on the idea that it is a name with a Hebrew origin.