How long has it been in your system? When did it start behaving like this? Was it moved from one area of your tank to another?
It is usually caused by some stressors such as excessive lighting, some rapid chemistry changes, a pest or sometimes it happens after feeding. The most common cause is probably too much light (or a rapid change in lights). Also some zoa can tolerate changes in conditions a lot better than others so they may not respond to stressful situations the same way as others do.
The last time it happened in my tank was when I moved a coral that was shading out a zoa frag and it looked the same as yours when it was hit with all that extra light. I moved that zoa frag to a lower par area of the tank and it bounced back.