Okay – My 2cents… which, since its free, has no warrantee…
I assume you have a Merc (based on the boat you were selling) and it has a “Flow-Thru” Hub. The piece is probably just “stuck” on. It is the backing plate that the threaded nut pushes against through the prop and plastic “break-away” insert to insure a balanced and secure fit. Just take a small hammer and tap it… it will likely “pop” off. Grease your shaft liberally to insure this doesn’t happen again. Or, just leave it and put the flow through hub back together and leave it in place – no need to change it.
Hope this helps, Barron – sorry to hear you had prop issues.
By the way… lubricating shafts can lead to some slippery fun… Have a grteat weekend!