Chalk brood. It's a fungal infection where the larvae basically turn chalky and are removed by the house keepers. It occurs more often in weaker hives. You don't have to worry too much about it. It usually clears up as the strength of the hive increases.
What you can do is inspect the brood to determine the extent of the problem. If there are not too many bees covering the frames, then you could decrease the size of the hive until the numbers have built up sufficiently. Look at how much brood there is. You might have a poor queen, or poor foraging conditions. You could add brood frames to increase the population, or requeen. I never have much success requeening myself, but you have a better chance with a new queen if you are on good honey flow.
If the hive looks otherwise healthy and only a few chalkbrood mummies, then you could just leave it to clear up itself. You might want to also check if there is a reason the hive is a bit weak, like another disease or pest.
I'd also put a shelter over the hive from the rain. It might be a coincidence, but I had my worst case of chalkbrood in wet weather.