All I can tell you is that is will go down in time but everyone is different. Mine went down pretty quickly once I got below 15mg but others took longer. Moon face is annoying but of all the problems you can have with lupus it is one of the least. It is not a permanent side effect and it will go away.
Many think it is edema but what prednisone does it to change where you store fat. Unfortunately it causes a moon face, prednisone belly, and camel hump but these are all temporary although getting rid of a belly, if you have one, is the hardest I think.
Bill