Feather duster growing extra tube?

[Dolphin1]

Hi - I have had this feather duster for a couple of weeks - this morning I noticed a second tube is growing - I can't seem to find any info on it (apart from when the worm leaves the tube). Is this normal or is he not happy where he is placed - looks quite fragile though and not very well attached to the main tube?

 

[Dolphin1]

mmm - maybe its actually the top of the original tube broken off?