looks like a fire worm. CAn be a bad thing some people like them. They eat dead stuff, snails, corals, and other things. I would take him out (with pliers or tweezers). They do eat corals so if you leave him in there he probably eat anything dead or not moving in your tank
It's a bristle worm, but not a "fireworm" (a common name for specific speces that have a firey red color and is known to eat some corals and/or clams). Bristle worms (aka Polycheates) are normal organisms in a healthy reef ecosystem and I wouldn't remove it IMO because chances are, it does more harm then damage (if any).
The vast majority of polychaetes are harmless to corals. I would just leave it in. I have probably 100 or so bristleworms in my 6 gallon nano, and my tank does fine. They don't eat snails, corals, or anything but detritus.
Mine eats my snails. The spin a web of mucus and suffocate the snails then move in for the kill. I have tons of snail shells that are empty cause of my guys and they look exactly like the ones in your nano
Have you seen them do this, or are you speculating thats what "spun a web of mucus" because you saw the bristleworm eating the snail (which was coverred "mucus" at the time)?
The worm looks like Eulalia viridis. From "The Modern Coral Reef Aquarium" Vol.3 it states " The basic diet for these bristle worms is dead or dying organisms (Vine, 1986), which can create some concern among aquarists. Although some species look very dangerous, they are quite harmless."