Large crack in wall under window cill

[Jodielydiaf]

not sure where to post but i need a bit of help, ive got a problem with mold in this room which i washed just 3 days ago and this was not there then. Can anyone tell me what this could be? and what needs to be done about it? Or How it could even appear so quickly? (Video not very good) but you get the idea.

 

[tony1851]

As you have done, many posters with 'cracks problems' show detailed, close-up views of the crack itself but not the context. Views of the surrounding areas, both inside and outside are usually necessary.
In the absence of a fuller picture, you will get no meaningful responses.