How to keep a green smoothie from getting 'frothy'?
As Catohound already mentioned, it's a symptom of using a high speed blender.  You may find this technique useful.  


  Tcrn the blender to its lowest setting and then slowly ramp it up until the top of the liquid just starts to circulate (usually around speed 4 or 5 on a Vitamix). As you run it at this low speed, you will see bubbles coming out in the center. 


My wife and I do this with our Vitamix and it generally gives good results.