I'm sorry, from the pic, unless I'm not seeing what I think I'm seeing, there is a white film going all around the edge of the skeletong. Is this what you are referring to.
I hope this works, take a kite string and hold it above a table and lower your hand to the table. Watch the string as it falls and watch the way the string wraps arounds on top of one another this what the substance looks like. As the coral opens up for the day some of the white stuff lostens up and drifts off but the majority still remains.
This would be an excellent way to describe mesenterial fillaments (acontia). Acontia is a defense mechenism of corals where their innards come out of the mouth and digest their prey from the outside in. This is not usually a problem, and I have no idea why it would continually do this.