You have a private mouth that's a machine. But you have to keep your machine mouth private because it would scare all the non-machine people around you.
To make sure they can't tell what's machine and what's human, you dye (だい) the mouth to make it look like skin. You find dye the same shade as your normal mouth and just slap it on there.
Currently there are no similar looking kanji which exist in the WaniKani learning system