worm

What is this???? About the size of a nickel Disappeared when I turned lights on.

Greetings! So I am fairly new to salt water aquaria -- I kept a planted freshwater for a few years -- and things are going pretty well so far. A short time ago though I discovered a few new "guests" in my tank. They look to be small tendrils/worms encased in sand tubes, which based on a bit...

I just got home from work and wanted to check the tank out before I go to bed and noticed this wierd looking worm. Can anyone tell me what it is and if it's good or bad?