These mushi are created when living beings are exposed to the light emitted by the Ginko. When a living being is exposed to the light emitted by the Ginko for a long time, they turn into Tokoyami. This is also how the Tokoyami came into existence. At night, or at dusk, it will emerge from its hiding place to feed on weaker mushi. As it does so, it gives off a silver light. Tokoyama will also attack humans - if attacked, the human must remember one thing, but sacrifice all other memories to the mushi in order to get away. If the victim cannot remember anything, he or she will be consumed.
If one is ever swallowed by the Tokoyami, first one must keep one eye closed and the other open (the open eye will then be taken by the Ginko). One may escape from this mushi by using this method, but will only be able to remember one thing, such as a name, even if it isn't personal.
Prior to the start of the series' story-line, a 10 year-old boy named Yoki was swallowed by the Tokoyami. He managed to escape, but only after being exposed to the Ginko thereby turning his hair white and his remaining eye green. Yoki was only able to remember one name, "Ginko", which was the name of the mushi within the Tokoyami. He then went by the name Ginko and became the protagonist of the series. (We see all this in episode 12 of the anime, 'One-Eyed Fish', which is set in an earlier time than the rest of series, and, along with episode 26, 'The Sound of Footsteps on the Grass', is essentially Ginko's origin story.)
It has an affinity for Light.