I have discovered how to reset a mob and return it from being in the grey.
If it is a neutral mob, you will need to engage it in combat,
a silence or a taunt is good for this, if it is low-level (to avoid 1-shotting it)
If it is a hostile mob, you only need to get close enough for it to target you, and then fly out of it's range, or run away.
This trick is also extremely easy for Rogues(Vanish), Hunters(Feign Death), Mages(Greater Invisibility), and all Night Elf characters(Shadowmeld) because once you start combat, you only need to leave combat in order for it to go back to the red or yellow.
Once the mob returns to the red or yellow, you are free to kill it like normal, or have a party member waiting to attack it as soon as it resets.
thanks to Khorium for editing for me