It's odd how two corals can live side-by-side for years and never bother each other, then, all of a sudden, turn on each other.
Last night I caught one of my corals attacking a small bunch of yellow polyps that had never bothered it in the past. It threw out it's stomach, engulfed the polyps, digested and ate them away!
Here is the scene of the attack. You can see the larger coral to the right and the yellow polyps to the left:
A closer shot shows that the "innards" came from only one attacking polyp of the larger coral:
The digestion attack is under way and the entrails are visible as they cast their net over the polyps: