As someone who's never heard the name spoken before, all I see is "moan." I understand it's pronounced mo-AN-a, or something similar, which isn't bad.
You never know, Disney may make it more popular. I always thought
Elsa was a weird, ancient-sounding name and Frozen kinda changed my mind but not enough to use it. Then again,
Elsa was kind of hovering in the top 1,000 (in the US at least) for years before Frozen even came out.
Moana may turn out to be one of those names people have never heard of but like it the more they hear it. However, I don't think it'll be nearly as popular as
Elsa. So if you want to use it, I think you're safe!