What's this white crust in my kitchen pipe threads?
Without the photo it could be several things , the first that comes to mind is not normally used on those fittings normally pipe thread sealant or pipe dope , those threads normally have a compression ring of rubber or plastic and should never see any water that’s why I put it first. Pipe dope turns hard and crusty after a couple of decades and is be easy to break those nuts on the tail piece and trap if dope was used.
Other things that come to mind when there are leaks quite common in the trap is calcium or lime both are white and crusty wince there is buildup but for that water to get out to the threads the seal would have had to fail.
So I believe it was pipe dope in a location that it should not have been used. A third possibility is plumbers putty that is used to seal the drain flange to the basin but it should not have been used on threads.