I have many in my tank and never had a problem with them.
Did you feed the nem the night before? The worm might have been attracted by the food in the nem, and the nem tried to close up on the worm and found he wasn't so nasty of a nem as he thought. "that will teach him"
If you see the big ones around there to much, it might be time to get the big ones out before they get a liking to the wrong stuff.