worm identification

As attached. May i know what kind of worm is that? Should i remove it? It is found at night after light off. Saw it swimming frantically before the video

How worried should I be about all of these worms in my sand bed?? This density of these critters is scary!!

So I have heard of Detritus worms, but these are high up on rock work, and just on the rocks all over the place. Can’t seem to find a picture that looks the same. Thanks!

I was readjusting a little log we have and when I picked it up all I saw was a stomach? Part. I literally jumped back, it surprised me. Does anyone know what it is and is it safe for my fish, crabs, snails, and coral/mushroom corals? They're my main concern obviously and I just want to make sure...