The ability doesn't say whether it needs to be activated. However, the "a 7th-level Dark Moon Disciple can disappear into the shadows, gaining total concealment" part seems to imply that it isn't, per default, always on. In order to keep things sane, I'd say it takes the usual standard action to activate. It's still pretty nice, since this ability has no clause as to when it ends except for the prerequisites (no natural or magical daylight).
Bottom line: no, you're not 'considered' to have total concealment, since that is not what the ability says. But you can give yourself total concealment. As per usual for (Su) abilities, this would take a standard action.