It doesn't mean anything. It has been used as a pet name for girls, intended to give some kind of cuddly associations. ("Baby talkish"). However. In the 1930:s - 1940:s there was an authoress who used "Moa" as a nickname, Moa Martinson (real name Helga Maria Martinson). She was politically radical, and because Sweden was so severly plagued by leftism in the 1970:s, a lot of socialist parents who liked her books, were inspired to give their daugthers this name as a genuine name. Today, though, the name's political image has more or less faded away, so anyone might use it.