When I was trading in my 07 Prius on a new one a little over a week ago the sales person asked me if I would be interested in a Prius C, hard loaded including nav for about 25,000 coming in the middle of March. I wasn't interested in one and ended up buying a leftover 2011 at 0% interest.
It's to distinguish it from the traditional sunroof which you could tilt and slide but offered no additional lighting from above. A moonroof is essentially a glass sunroof.