The 1881 census was as at the night of 3rd April.
If this was your Alexander on the crew list for the King Coal, then according to the Hartlepool Mail 4th April 1881, the King Coal arrived at West Hartlepool on 3rd April, from Hamburg.
So I looked at the 1881 census again and the King Coal was in there at West Hartlepool Dock, RG11/ Piece 4912/ Folio 165/Page8
and there's an Alexander Edington, as per the crew list working as a fireman, but he is down on the census as being 46.
Its either an error on the census return or an error replicated in two separate crew list agreements which both had a birth year of 1857 recorded.
I'd be inclined towards a census return error, but its not my family, so only you can decide :-)
Boo