NASA has just released a photo showing a part of the InSight Mars lander wet, like a cardboard box that came in contact with water (see the picture). What is that liquid?
The Instrument Deployment Camera (IDC), located on the robotic arm of NASA's InSight lander, took this picture of the Martian surface on Nov. 26, 2018, the same day the spacecraft touched down on the Red Planet. (Source)
This is the object that looks like a wet cardboard box.
Regarding the theory of reflections advanced by somebody in a long answer,
I have to mention that the same image shows numerous reflections (see the picture below). None of them looks like a wet cardboard box and besides this I have never seen a similar case, when a reflection created the optical illusion of a wet cardboard box.
Using two more pictures posted by NASA, I made a gif that you can see below. It is evident that the theory of reflections, so insistently advanced by a user, is wrong. It can not explain why there is no change in the shape and size of what appears an area of cardboard that came in contact with a liquid. It is evident from the animation, which stretches across Sol 0 and Sol 1 (Day 0 and 1 on Mars, since landing), that there are numerous reflections and shadows that vary from photo to photo while nothing changes regarding the wet area.
Animation made of 3 pictures taken by one of the on board camera of the InSight Mars Lander on November 26 and 27, 2018. (Source)
Update, December 1, 2018:
Animation made of all available pictures taken by one of the on board camera of the InSight Mars Lander during 5 martian days. (Source)
Still no change in the size and shape of the wet area. There is an evident shadow above the wet area that changes. It is generated by the black object, in the middle of the image, which moves.