u have the 'kill mind invader quest' and it doesn't let u have 2 similar quests in bag at same time
delete that one (or complete it) and it's done.
this.
You didn't kill Mind Invader last time you did that quest, so it remained in your quest log, preventing the ability to complete the same quest twice.
Since its been a week since you spawned it, the mind invader is long gone.
Trash the "Kill Mind Invader" quest, then turn in your Lochmur Kid quest and another Mind Invader might spawn.
Kill it to complete the quest.
If you're in desperate need of the 50 faction merit it gives for killing it, ask a faction mate to squad you and do the same quest. Hopefully their quest will spawn the mind invader also, and (I'm pretty sure) you can both get the kill if you're in squad.