It's a Eunice worm, definitely don't want that in your tank.
I agree with sugar magnolia. Though I wouldn't flush it or throw it out. There are a handful of people on here that are actually looking for one to keep in a species tank and they'd be happy to have it. Or, you could dedicate a tank to it and grow your very own real life sea monster
If you know someone that wants it let them know to get in touch with me.
+10 to the responses above!!!
They go after snails, peppermint shrimp and hermits and dusters!
If you have ANY of these in your tank get rid of it!!!
Selling/giving away isn't allowed in any of the main forums.