We have a bathroom in a detached structure from our house. We purchased it and have not used this for 6 months.
I just noticed this powder on the walls and I touched some of it on the wall and it just crumbled like baby powder.
It is also on an adjacent wall. There's also white stuff around a hole in the ceiling.
It is also under the sink...around the pipe.
Note this powdery material has build up on the wall so that it is 1/2" or so thick.
I don't know what all of this stuff is and whether it's the same substance in other places in this bathroom.