I have lived in a rented apartment for several years
now. One day I came home to find bathroom tile in the
bottom of the bathtub. When I inspected the wall beside
the window where it fell out, I found that the plaster
had split (delaminated?) with a thin layer still stuck
to the mortar(?) and lathe, then there was a layer of
very fine white powder, and then the back of the tile
had remnants of another thin layer that had come loose
from the layer of fine white powder. In some places, the
tile mastic was still holding THAT layer to the back of
some of the tiles.
Can someone tell me what I'm seeing here? I could have
imagined the plaster getting wet and coming loose from
its substrate, but why did it split into two uneven
layers with white powder in between them?
Any suggestions on how to get something to replace the
missing plaster and get it to stick to whats already there?

TIA

Rick