Anybody know what caused the grey mark on the wall?

Is it a uniformed pattern and a straight line? I see some near the window too. Could be dry wall marks if they didn’t do enough to smooth it out or cover with enough primer. Just my guess… you can Google this

也许那里墙面没有远看那么平？有灰色的地方可能是上下两块材料交界处，凸起来或者凹下去，灰尘留在上面？
还有一种可能的情况：上下两窗吹进来的灰尘在灰色处对冲，这样也就解释了为什么靠近角落的灰色更深。

不是渗水发霉了 吧？