Those slugs are very common these days. They just showed up a few years ago and now they are almost everywhere. I call them slugs too! Great algae eaters. Largest one I've seen was an inch long. Anyone longer?
The Nassarius were probably mating. That's more likely than fighting. You know... make love not war
Most worms, including Bristleworms, are very good for the tank. They eat detritus and dead animals, usually overnight!
If you find a rare one eating coral it's easy to catch. Get some bridal veil material from the fabric store. Wrap up a piece of frozen brine shrimp, freeze dried shrimp or raw shrimp in several loose layers of bridal veil tied with fishing line that is long enough to extend out of the aquarium. Watch the bristleworm go into the material to get the food and pull it out of the tank. It will scrunch up and be caught in the material. The bristles are really irritating so don't handle the worm.
Hope this helps
Edited by Mark Peterson