Just a summary; vermitid's are not typically harmful to other animals, even when they are around in large quantities. They usually only attach to bare rock or coral skeletons and don't have any stinging cells that will bother other animals. They can clog pumps and powerheads, drains, etc. though...
The article mentions some hermits helping to eat them, but I've personally never seen this. If I need to get rid of them, I usually just crush 'em...