This is a video of Youtuber gurutek cleaning his saltwater aquarium to reveal the giant predatory Eunice worm that had been living in the sediment. Apparently the 1.2-meter worm had been living in the tank unbeknownst to him for a year before he first spotted the thing, and only saw the thing three times in the span of the next year when it would come out at night to feed on his coral colonies. Personally, I would have stabbed that thing with a trident and flushed it down the toilet like a normal person. "Amen to that!" Okay, well technically you're not a normal person, Aquaman, but I do appreciate your support. Hook me up with your mermaid friend?
Keep going for the video, which you should probably not watch if you get the heebie-jeebies easily.
Thanks to Frankie, who agrees finding worms somewhere you don't expect them, in general, is bad news.