A condition of brood
Bald Brood is the name given to a condition of brood where there is a straight line or small patch of developing brood that has no cappings. It is caused by greater wax moth larvae tunnelling under the sealed cappings of worker brood. I say worker, because I have never seen it in drone brood. I have seen reference to lesser wax moth being a cause, but I have only seen greater wax moth tunnelling.
I assume what happens is the bees remove the silky tunnels, leaving the worker pupae exposed. They are always a few days away from emerging, often at the pink eye stage. To the best of my knowledge the bees emerge. I assume the bees don't reseal the cells because there is no pheromone prompt to do so. The lips of the cells are often slightly raised.
Bald brood is not serious, although it is probably more common in weak colonies. There is no treatment, but keeping colonies strong is always advisable.