Photos of the poop may be helpful.
Do they have any other symptoms besides the mucous?
If they do, then treating symptoms with an antibiotic may be in order. Do keep the nostrils wiped clean. I'd still trim some of the facial feathers so they can see well and so they are not getting all...
How much ventilation do you have in their housing?
Have you added any new birds recently?
Try trimming some of the feathers from around the eyes and face, see if that helps keep the nostrils cleared of feathers. Then see if the nasal discharge continues. Hard to know if getting feathers stuck...