from what I have read, fish to fish have different results and how much you feed plays a part in likelihood of leaving the shrimp alone.
It has to be pretty boring in a glass box and the triggers are a pretty intelligent fish so I am guessing boredom would play a part as well.
I had a Blue throat for a while and found the existing shrimp and such in the tank before he was placed in seemed to be left alone but anything new usually disappeared so if it's part of the environment when the nigger was added have a higher chance of being left alone but anything smallish and dropped in are likely to be seen as food.