Is it above the tail, or under the tail?
In this month's issue of Backyard Poultry there was a q&a about something LIKE that, under the tail, only the hen had a white discharge. The columnist called it possibly "vent gleet" which he said is a kind of fungus that can be treated with iodine spray. The other possible cause was a disease mostly common in older hens (I don't have the mag right here or I'd quote it).
Are they picking on each other?