I checked on my Coturnix Quail this morning and one of them had laid a white egg. When I picked it up it was super soft...quishy. And beside it there was a white membrane...looked like a piece of a balloon. One of the quail in that pen just started laying a couple of weeks ago. I don't know if this was from her or if a different female laid it. Its the first one I've seen.
Soft White Egg
They may need more calcium in their diet, one of my quail laid a few eggs like that so I gave her extra crushed oyster shell and her eggs went back to normal in a day.
I keep a small bowl in their pen almost always and sometimes sprinkle a pinch on their food for the few females that don't always eat it. But if it was just one egg it's probably fine, maybe that female was just having a bad day.