iPhone X ambient light sensor

What thing? All I see is a lot of finger smudges, no dust. There is certainly light reflecting off the screen. It appears that you're pointing a light at that portion of the screen.

It looks fine to me.

If you look at this page from the manual here: iPhone X Features

and reproduced below you may find the answer. It looks to me like you are pointing to the TruDepth camera

Which I one?