Security issue on blocked screen in High Sierra?

Your picture shows "burn in" or "image persistence" where the physics of the liquid crystal elements that are held at one voltage level for a period of time tend to not relax entirely when that voltage changes.

This affects the polarization of the backlight and you will see ghost images.

Short term, you can reduce the backlight level so that it's not letting as much light through, but long term you can use a screen saver or just select a higher cost display that shows less of this inherent characteristic.

Other display technologies have similar effects, but they can be more permanent with CRT tubes or OLED displays. LCD generally recover over time, but can be permanently marked and need to be recycled or used in less discerning uses once they are used past the design life.

• https://en.wikipedia.org/wiki/Image_persistence

Image persistence can be reversed by allowing the liquid crystals to return to their relaxed state. In other words, turning off the monitor for a sufficiently long period of time (a few hours or maybe days) will relax the crystals and eliminate image retention. For most minor cases, simply continuing to use the computer as usual (and thus allowing other colours to "cover" the affected regions) or turning off the monitor for the night is more than enough.