Its so good but it seems to be the good head sketch.
Put more shadows and volumes to it and itll be nicer.
Also some green things looks like a snot drops :3
And also it is not even nearly enough to use bevel and emboss
to make shadings realistic.
Make some volume and put some texture by your brush.

The flat doesn't look too good, but that's not what matters. It looks good ingame, and THAT'S what matters. 9/10
I don't think there's too much to fix, but the top looks a little dull.