hitchhiker worm

Hi! I noticed a nasty lil worm thing moving, inching its way out of a hole in the rock. Obviously I was curious to see what the heck it was so I turned the tank light on and it shrunk back in the hole. I'm new to saltwater tank keeping so I just want to make sure this isn't a harmful parasite of...

I see these stringy things coming out of the sand all the time. I believe the worm stays under the sand and puts the strings out. I saw it on the glass and tried to take some pictures. Let me know if you can identify this. Thanks