That is a green mandarin goby (or dragonet). I have a couple, they're terrific additions to your tank, provided you have lots of well established live rock and sand loaded with copepods. usually can find em for about $20
Yep when your tank is aproximately a year old you too can have a mandarin. Need a well established tank with lots of 'pods. You can also find psychodelic mandarins. Similar only spotted http://www.aquahobby.com/marine/e_mandarin_2.php
I have one of those. Her name is Mandy. They do not do well in new tanks. Tank needs to be established for several months before you add one. Most end up starving to death in new tanks.