If you killed it you would know, they are very toxic when dead. Nems tend to move around until they find a suitable location. Depending on how big the nem you put in was you may not be able to see it. Very possible its still moving around the stack and is currently somewhere in the rocks where you can see it.
Also, if your lights are more intense than where it came from it may hide for a bit until it adjusts.
I wouldn't be concerned unless your tank gets cloudy or starts to smell horrible, Youll know if the nem has dies, theres no doubt about that!