My best guess is that it is the result of enteritis (from what exactly would be hard to say as several things can be responsible ) where irritated it will slough off.
I would not depend on DE (particularly if giving internally) to ward off worms (using it as a dust around the coop is good management but giving it internally will do nothing for any active worm infestation and some members have reported negative findings associated with it)... if you have free ranging birds that have never been wormed then worm them (ivomec EPRINEX > use the search function here to read more on that , dosage etc.) > use the "spot-on" method (applying to the skin)
I would offer a good quality live culture yogurt yogurt , free-choice daily and give a good allround supplement like AviaCharge 2000 (available online from McMurry or Strombergs) and ensure a good quality layer feed for the girls and fresh water > keep everything clean and well ventilated (often these are underlying problems)