It may be nothing more than a little of the egg-production lining coming away. But it may indicate worms. Do you know whether they've been wormed recently? There's a product called Flubenvet that you should administer every few months. It comes as a powder. You mix it carefully in their feed, and they continue with it for a week. There are no problems with eating the eggs.
You can also get herbal anti-worming remedies, but they're more of a preventative than a cure.
