Also, if the discharge is their droppings, are they frothy white? If the birds are adults, have they ever been wormed? Are the droppings dark green with white and non-frothy? Or just green and white, runny?
You'll want tot examine the birds thoroughly. Note whether or not they're underweight. Check carefully for parasites (mites and lice). They're nearly microscopic so check carefully against the skin and feather shafts, particularly near the vent. Sometimes you can more easily find their eggs than the bugs themselves. Let us know if you see any.
Also, do you rbirds have access to any puddles, ponds, algea water? Has it been raining there a lot lately? Any other signs? You'll want to watch them carefully for any signs of odd breathing, etc.
What type of bedding are you using? Do your birds free range?
Also Renee's answers are very well chosen and necessary. To most accurately help you, please answer all of these questions. Thank you so much.