Who is this a greek statue of?

That's not Poseidon. He has a beard. His image can be seen here: http://www.greek-gods.info/greek-gods/poseidon/sto...

It looks like the statue of Antinoos. It is at the same museum as the picture you posted (National Archaeological Museum in Athens). I am actually certain it is the same statue. Here's a link to it at a different angle.

http://www.flickr.com/photos/20945717@N05/39536908...

It is the head of the Marathon boy, a bronze statue from the Bay of Marathon c. 325-300 BCE in the National Archeological Museum of Athens see http://en.wikipedia.org/wiki/Marathon_Boy

The face looks like Alexander the Great but I could be wrong

I think this is Poseidon. The Greek god of Sea.