It is most likely a large meat spot, larger than average for sure. That is a piece of tissue from somewhere in the reproductive system. What the tissue is, we can't know. It could even have come from an ovarian follicle. It doesn't look like lash material that I have seen (and I've seen a fair amount), and I've never found any inside an egg (doesn't mean it couldn't potentially happen). But without knowing the texture, and seeing what it looked like inside, cannot say for sure. Hopefully it is a one time thing. I would keep an eye on your hens, make sure no one is acting off at all, signs can be subtle. It may have just been a glitch, which can happen for unknown reasons, or it could be an indication of a brewing reproductive problem. Really all you can do is observe, and wait.