Hi!

I just departed from EDDM and saw these weird white textures. Any idea whats causing this? 

I have ORBX  openLC Europe.

https://imgur.com/v15xPXm

 

It's called snow.

That's snow on the ground. The textures match up laterally. Either your weather add-on made it snow there or (more likely) for some reason you have "hard winter" (snow cover) textures at that location. The combination of ORBX Global, Open LC and P3d 4 gives you the rather jagged edges rather than the unnatural squares of snow that one sees in FSX.