Pest on coral

[Nihoa]

Something is creeping up this frag and building a casing as it goes. The coral is bleaching underneath but I can't tell if this thing is feeding on the coral or just growing on it. Any ideas what it is? I've looked at night and not seen anything sticking out the end of the casing and the casing itself goes all the way down to the base of the frag into the rockwork. Some sort of worm??