Sorry I just got back on the site:
It is kind of hard to tell where he lives, they can get pretty long. You will have to check the rock once you remove it to make sure you have him.
It is just like one minute before the critters adjust and go back inside the rock...so be ready to just grab the rock out. Then you can just pick up what you want to keep and toss it back into the tank, or remove the worm and place the rock back in the bucket and the critters will return to it.
No the worm didn't do anything to the starfish. It sounds like bad parameters, like high nitrates or acclimation too fast or starvation.
Your other critters will find a new home or return to the rock once it's replaced. So don't be afraid to disturb them.