Crystal formation on new plaster?

To provide an update: As feared, the shaft is in very poor condition, caused over the decades by moisture always seeping in through the window. First, it was not properly sealed and the window mortar was barely present. Second, the shaft was never treated, painted, plastered, or even cleaned. It is completely green and there were buckets full of dirt inside. Third, not even the drain pipe was usable. It protruded several centimeters above the ground (so nothing could flow in) and was also clogged with all the dirt.

The shaft will now be properly renovated and the connection to the window correctly done. Then there should be no more problems inside.

Thanks for your help!