Two possibilities.
1. Albert Greener was a successor to W.W. Greener of the London gunmaking family. If an authentic Greener, your shotgun could be worth a pretty penny.
2. It's a Belgian copy. Unfortunately, the Belgians copied Greener-style guns and intentionally put misleading names on them designed to make people THINK that the gun is the real mccoy. One of the known fake names used on guns was... Albert Greener.
In order to determine which you have, you need to start looking for the proof marks.
A "faux" gun made in Belgum will have Belgian proofs on it. A gun made in England by the Greeners will have British proofs.
That's no guarantee that your gun is either authentic or fake, but it's a start.
My guess is, though, that it's a Belgian gun. I don't think Greener marked many of its guns Albert Greener (if any) and the Belgians turned out huge quantities of faux guns.
According to Wikipedia, Greener will authenticate or disproove a Greener marked shotgun for a fee.