Too small for mouse. Even dehydrated wild mice leave droppings at least 5mm long. 
There are different strains of chalkbrood, I have seen these smaller more dehydrated and darker than yours pictured in my mating nucleus hives in Australia. Also while the bees are removing them, sometimes they break up. So what he has measured is most likely a partial chalkbrood mummy. I am in no doubt as to what I am seeing
It’s actually the stage of the fungus that changes the color:
https://www.ars.usda.gov/northeast-area/beltsville-md/beltsville-agricultural-research-center/bee-research-laboratory/docs/chalkbrood-disease/