Was looking under my bike for a sqeak and noticed something wrong. Believe it is the oil seal from my rear differential (finial gear). Anyways, what looks like a washer is hanging out loose on my axle. Looks like I could put back back in the housing, oil seal appears under where this metal washer should go. No oil is leaking, just the metal cover/washer is loose.

Is this a common problem. Can I just put this cover back on and give it a few taps with the hammer?