marie I've never seen anything like it.....

I have seen in the past though small shell builders in acro colonies that live in the acro structure however they never eat the coral skin like you described what they do is feed by casting a string and these are small like 2mm wide.

Marie, did you purchase any new corals recently that may have hitchhike from there. Sounds like something you just encounter, possibly contact the vendor and see if they are having any similar problem. If you noticed any new damage, how about removing that particular piece if possible? Regards Ken

I first noticed damage done by this guy a year ago and at that time I wanted it out of there but after living with it for a year I'm not as bothered by the damage. As long as the coral it's munching on is healthy it appears to be able to grow quicker then its being eaten.

The tube it builds reminds me of an egg a chicken lays that doesn't have enough calcium. White and kind of soft but still holds it's shape, it also has sand and debris incorporated into it when available. No strings, tentacles or webs ever come out of the tube that I can see and the tube travels in multiple directions...it has lots of entrances/exits.
All this info I got from when I first found it last year, I have only recently just rediscovered it and the tube so far appears to be coming directly out of the rock

That is sooo weird Marie. And, you do just learn to live with stuff, as long as it all looks good, and is growing faster than this creature can eat it..then all is good...in a way!