beak grinding

My cockatiel has seemed a bit off lately (always wants to be in the dark, less vocalization, ect) and I noticed this odd thing he was doing with his beak today. I have attached a link to a video of what he's doing on the bottom of this post. He doesn't seem to be grinding his beak but more...

I know that I read SOMEWHERE that beak grinding is a sign of contentment. But Handsome is doing it pretty much constantly. If he's not eating, he's grinding. Anyone know if excessive grinding is a bad thing? Or is it just more of a good thing?