I had the same thing. Actually have thread a ways back about it. It was the consensus that it was some kind of feeding tentacle. I left it alone and mine were way longer then that and I had about 4. Never did anything and just kinda went away one day.
"When creating an aqua scape, strive for the beauty of imperfection, as its art and there is no wrong or right, it's left open for interpretation" James Findley