This might well be "shell rot"
One thing is sure, its not down to lack of calcium in the crays diet.
coz they donāt need more than trace amounts.
They get all their calcium from the water around them.
Iād treat immediately with something like Melafix.
Iād treat it in a Q tank or a bucket if you do not have the former.
Keep treating till you see a reduction in the lesions.
Then transfer to the (freshly cleaned and disinfected tank, donāt forget the deccor and substrate.
Another option would be salt.
Use 35grams of salt in 1000ml of water. Or 750 of tank water and 250 of boiled water
Place the cray an a big enough container and treat the cray once a day for 10 minutes for 3 days.
Hopefully one of these will help and the rot will either vanish with the next moult of reduce.
Another option might be bad water quality or low quality light.
It may well be worth popping over to crayfishmates.com.
Iām sure Crusty will put you right, even if you say TYLOR sent you.