Crystal formation on new plaster?

Hello,

at the moment a basement granny flat is being built here and the interior plaster was applied about a week ago (it is still not completely dry in some places). Now I have discovered a spot with crystal formation under a dormer window (which was also only recently installed along with the windowsill) (approx. 30 cm x 10 cm). Picture is attached.
How does this happen? Is this a problem?
I also checked with a flashlight if it sparkles anywhere else, but it is only at this spot. You can also see that the crystal formation is very high in most places. I would estimate 2 to 3 mm at the highest point.
Unfortunately, I no longer know exactly what kind of plaster was used.

Cause: Moisture

Cause of the moisture: I guess: light wells were screwed on from the outside and in the process the outer basement waterproofing was perforated? Or during window installation the sealing at the transition between basement exterior wall and window was not "connected" or not made?

It is an old house. Approximately at that spot, the old plaster had also been damp. The workers said that this was due to the shaft at the window, which is not properly sealed or not angled, and water can penetrate under the window there, but he would fix that later. Isn't something like that done before plastering? Is the plaster now ruined at that spot?

If it is rainwater, it would of course be good if it could seep into the shaft (holes on the side facing away from the house) or would not even rain into it in the first place (cover the shaft with a translucent roof). But I rather don't believe that, I would guess it's moisture from the ground outside. Or was the cellar possibly already excavated and renovated?

The basement is renovated. Only in that spot it is still damp. As mentioned, when the old window was removed, it was also seen that the mortar underneath was completely crumbling at that spot because moisture had always penetrated there. From the outside, you can see about a 1 cm gap between the wall/window and the shaft. It should be sealed and angled, the workers said.

Google "efflorescences"