Over time the pixel switching elements can burnout. Often during the manufacturing process the glass plate may not get enough metal-oxide deposited or the PNP/NPN silicon or color filter material was not properly laid down. So the given group of pixels in an area of the screen wear out sooner. There is little you can do to fix this type of failure as its age.
A second issue that can create this the defuser sheets which is part of the backlight system get wet which I see here the signs of along the edge of the image area.
I would live with it as its not that bad. It is not likely to get worse as long as you don’t get your system wet again.