What is creating the tube?

[DadE02]

I have recently started seeing this tube growing out of the bedding stone of my aquarium.

Each morning it gets longer, but I don't see anything in or around it?

My kids have been asking me what could be creating this thing. Any ideas?

 

[Sniperhank]

It's been awhile...it is a worm that is making a tube out of your substrate. I want to say spaghetti worm.