there are 1000's of worms that u can just get hitchhiking and millions in one area in the ocean...if u can catch it then try if not u shouldnt need to worry. i have un ID'ed worms in my tank that swim around at night and they have caused no harm. and dorsoventrally means um...i cant really enplane it other ways....i guess top to bottom....but not really. sorry about the scientific term...and a bio major i tend to turn them out...if i ever confuse u in the future feel free to pm me about the termno idea what dorsoventrally means but it went side to side, just had its head up near the top of the water.. and im not sure about the sides of it.. it kinda looked smooth but then the sides of it looked as though it had ripples...
usually the deposit feeding bristle worms dont swim...their larva stage will but typically they stay on the bottom when mature.answering your first question about your caterpillar looking thing.
I do believe you have a bristle worm. I have a couple in my live rock and they haven't been a bother to my tank. I do believe one of the reasons they show up is from over feeding and they will multiply so try feeding less. I cut down on my feeding and don't see them anymore. But don't worry cause they won't harm your tank