Why is there a stain beneath the screen?

Hello. On the display of my smart phone (Blade A601) a transparent black/gray appeared. It is present when the screen is on; when it is off, there is no difference on the phones surface. Four hours before noticing it, the phone was alright. I think it might be water, but it is improbable.

If you could give me a possible diagnosis, that would be awesome. Thank you in advance.