Found an old post (2007) on Reef Central by @LeslieH:
"It is a polychaete, family Syllidae, subfamily Autolytinae, and possibly genus Myrianida. I have a picture of a very similar one called Myrianida pachycera on my museum's website 302 Found M. pachycera has been found Australia, Japan, Hawaii, and in harbors of southern California and are usually found on the sponges they eat.
You're right about the babies. The anterior end is the parent that is producing clones called stolons. When the stolons get large enough they will detach from the parent & crawl away. Members of the subfamily Autolytinae feed by piercing the body wall of their prey - sponges, hydroids, bryozoans - with a ring of sharp teeth then sucking out the contents. You don't have to worry about your zoanthids or corals with this guy."
Hopefully someone more knowledgeable can post some more recent info...
normally, the rule is if you find something that you do not know. Is you sump it. If you cannot sump it as it will still spread, then you remove it. Normally safe than sorry. Flush it down the toilet, throw it out on the lawn, freeze it and use it as hermit food later. Why risk you R1000 clams for one hitchhiker Muricidae snail?
That is if you cannot ID the critter.
If you bought, collected or received this critter while you knew you got no idea what it is, then you need to keep it alive as you did take ownership.