Noticed this today. It's a gravel tube spanning between two of my rocks. It's about 1/4" in diameter. Knowing I have a Eunice worm in my tank that pops his head up occasionally, I read up on it, and it seems like they do make these tubes and make webbing in various voids which I also see. I should probably catch him, but he doesn't seem to be doing any harm....yet 
Though I'd post this pic in case someone sees these tubes and was wondering what they have creeping around in their tank

Though I'd post this pic in case someone sees these tubes and was wondering what they have creeping around in their tank


