It actually is detectable since it is radioactive. In retrospect, I don't think this is the shadow object. However, it does cause lunar phases. It comes from gasses from inside the moon that are released due to radioactive decay and also from sputtering. Even NASA agrees that this is detectable.
As per its defined shape, a weather system could be as such for many reason. Look at a tornados et al. From above they certainly seem circular. In a low atmo environment it would be more likely to gather up and travel periodically, especially given sufficient time.