That piece shouldn't have much affect on the sealing of the axle seal. I believe they call that an oil slinger or something it sits on one side of the race for the differential, So replacing it would require removal of the trans/differential cover. I guess maybe it could limit the flow of fluid being pushed out of the spinning diff bearings but I can't imagine it has that much affect on the seal's sealing ability, especially considering it's only on the one side. I'd clean it off and re-inspect it after a little driving. If it's still leaking make sure there's no other problems like a scored seal housing or an accidentally ripped seal before you condemn the piece in the diff.