This is normal, and means the saline is doing its job - these are little mucus plugs that have been stuck in your lung. You can read about it in this discussion:
https://connect.mayoclinic.org/discussion/light-yellow-mucus-with-tiny-little-yellow-balls-in-it-is-this-a-flare/

At first everything about MAC and Bronchiectasis seem scary, but we can learn to live well and actively with it. Just remember to keep doing airway clearance with the saline, eat healthy and get some exercise (helps clear mucus and keep your lungs as strong as possible.)
Sue