Yes, XP for killing aliens in Lord of the Rings online...wait, no!
I mean if you happen to be slaying grey monsters for whatever reason. What if you still got 'appreciation' points for it (ie, appreciation from the local peoples), which you can turn in. But you can only turn in so much at a time and there's a long cooldown between turn ins, so you could only gain XP at, say, 5% or 10% the speed you could if you were actually fighting level appropriate monsters?
So you can still gain some XP, but it's preferable to fight things that are your own level.