I hate to say it, but first thing that comes to mind is AGY - Avian Gastric Yeast (formerly known as Megabacteriosis but it is in fact a yeast infection). The only thing that makes me thing that is the mucus from the crop flush. I hope it's not, as that's a tough thing to treat! I lost a budgie to it possibly due to one vets ignorance and an a-vets inability to pick up on the disease and by the time we did the crop flush, it was too late (a-vets out of town and they usually send stuff like that to a lab, even though they can do their own gram stains).
I hope Piper recovers! Keep warm and safe until you can get those results back!