I would not call spaghetti worms totally harmless. they will irritate zooanthids bad enough that they will die. I had a pretty bad problem with them in my frag tanks. I added a peppermint shrimp and took care of them. but other than that they are good little snacks for your fish that forage in the rocks (copperband,6 linewrasse,mandarin,etc)
They actually do look like little pieces of spahetti. I've only seen one bristleworm so far, there may be more. my coral banded shrimp is amazing to watch at night. he sits right in the current with arms outstretched like he's soaking up sun rays. really fun to watch.