Yeah, you're probably right. It's most likely just a rejected piece of detritus, and just a coincidence that it looks like a tadpole shape. I need to learn more about these creatures. They seem happy for now. I hope that they stay that way. I'm actively feeding them now, so we will see.
I think that I've figured out the genus of my worms in the video, Spiochaetopterus. Their are 2 tentacles to each worm and they use them to catch organic material that drifts by. They tentacles are sticky, apparently. And what you see in the video of the worm is probably about 1/2 of the total body length. If these aren't the actual species or genus, they look a lot like these.
Yeah, who knows what else!!!!
It's fun to discover new life in there!