Corals poop too. They eat things and have a digestive system. If you have a large colony of zoas, it's not uncommon to see several doing their business at any given point.
From the angle of the pic, it looked like poop to me. In fact there was a whole thread that went on for over a year of pics of massive coral poops. They are animals. Like all animals, they poop. They just happen to have one hole that they eat from and poop from. Imagine if we were like that;Yuck