The difficulty gauge is an approximate indicator of how hard an NPC is to kill. The difficulty gauge only appears for non boss characters. The difficulty changes as you progress through the game.
These NPCs are too easy and you will not receive any XP for them.
You should be gaining XP off these NPCs.
More difficult but certainly doable.
Getting intensively difficult, you will have a hard time unless you are well geared and have lots of hp. Wait until you level up once perhaps?
Requires good gear and lots of health to survive.