worm identification

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...