I would guess that the worm is just cleaning up dead tissue or happens to be there. No way that worm little ate all that tissue
. I've just never heard of coral predatory worms being a problem in a tank before. Either way, I think the key is to look at it at night - maybe every night 100 more worms are coming out and feasting on it. However, if that was the case I would expact the damage to be more spread out, not progressing from left to right.