Let it grow as it wants, as long as there is room for it there.

Or it is not actually a white mulberry.

bboy is right.

You COULD top the tree at deck railing height. Sometimes this will promote energy for lower branches. Young healthy trees often recover from this. Most folks hate it when their young tree looses a central leader though.

What amount of sun does it get? Less sun more likely the tree will grow tall and thin like a forest grown tree.

What kind of maples do you have nearby? They're the most likely parents. How big are they? I have a volunteer white ash growing near my shed. Its way to close to the shed but I've let it go until this or next year when I'll have to cut it down. Too much longer and it'll be a liability.