I know this is an old thread, but I just want to add that I also have one of these in my tank. It's been slowly killing a blasto colony that we've had in the tank - healthy - for over 4 years. Every time I aggressively vaccuum it out/scrub it off, it comes back within a day. I collected a glob of the soft "tube" and put it under my scope - it just looks like a pile of sand. Since it's near the top of the tank, not anywhere near sand, I've come to the conclusion that it's burrowing into the coral and that the sand in its tube is actually crushed coral skeleton. Since manually removing it has failed numerous times, my next approach is to super glue the whole area and hope to smother it. Ofcourse if it burrows, there's a chance it will just burrow a new opening next to the super glue section, but I've got no other ideas left. Crazy to me how dismissive the forums are - everyone is convinced this doesn't really exist. But if you've got it, you know.
I have had the same thing for at least 10 years in my small reef. The tube substrate is silky like a spider web, but often has lots of small sand type particles imbedded that give it a gritty texture. Diameter is around 3/16" and length can be up to 2" or so.
I'm not sure if the tube material itself causes the death of coral tissue, or if the worm making the tube is actually feeding on the coral flesh. Tubes sometimes appear on live rock or sand where there is no coral in the vicinity and they are always constructed during the dark hours. They also show up for a week or two, then typically disappear for a few months before showing up again. I suspect it may be a worm in the Eunicid (or related) family like my tank buddy, 'Bob' here:
Here's a R2R thread from a while back on the same topic:
https://www.reef2reef.com/threads/e...ptoms-how-do-you-know-if-you-have-one.175219/
My latest casualty was a large Blastomussa vivida that has been damaged from tubes three times over the course of a year. This last time about 2 weeks ago, only 1/3 of the polyp remained and I decided to move it to a brand new location where it is recovering nicely.