Agree with the others, its Ick.
And you may be wondering how this could occur with the UV working for the tank.
UV "only" kills those planktonic phages that pass thru the unit, it will have no effect "what so ever" on protozoa on the fish, and those in the substrate.
And in the case of ick, most are not motile enough to be swept into the unit.
I don't know if your crawdad is locally wild caught, or not, but they have been thought to be carriers of Ick and other maladies (even though they do not succumb to the disease themselves), so any crawfish put in the tank should receive the same quarantine given to any other tank resident.