think it's just a rust proofing for storage to make them look nice on the shelf...would you buy a brand new tow ball with surface rust on?, clean it off well, fine wet n dry or scotchbrite is good, then clean off with a bit of meths on a rag before hitching up ...you want to prevent pad contamination on the hitch.
Always worth keeping a bit of wet and dry paper in the van as well. When we went away to dorest this year I forget to replace the towball cover after we had set the van up and after 8 days of damp and rain it was a lovely rusty colour