Great work!  The only thing that's bothering me is the horizontal movement of the nebula.  In space everything is so far away, that it would all pretty much be moving in the same direction and at the same speed in relation to the player.  And as an enemy, it would likely need to be close which would make it extremely huge.  Like light-years huge.  

But given the fact that it could be some other entity altogether, none of this may be relevent...