That's really hard for me to ID but if it is a zoa eating nudi they are very easy to kill. From my experience, the surrounding polyps would not open which doesn't appear to be the case here.

Freshwater dips from 30 sec. - 2 mins. will kill them, I was successful around the minute mark. Your zoas will be fine, but there may be more, so keep on the lookout. I always fresh water dip zoas for a minute or two. Nowadays it would be so painful to dislodge every colony just to dip. They usually take on the color of zoa they are eating, which makes them somewhat difficult to detect.