Doubtful they are just on one rock..and they multiply like bunnies

Unless they are on the glass and you get lucky and find one, they are otherwise hard to see as they take on the color of the particular zoa they are eating, and do a lot of damage when your lights are off.
You need to figure out how to get that rock out of there, or you won't get rid of them..and eventually won't have any zoas left. Zoas are like candy to these guys and I believe the only thing they eat
There is no way to treat for them in the tank, dipping is the only way to kill them all off (and either scraping the eggs off, or sacrificing the polyp they are attached to.
Here are some pics of what they look like, including a pic of the eggs:
http://zoaid.com/index.php?module=Gallery2&g2_itemId=415