Yep, and those suckers reproduce like rabbits. Brian had literally thousands of zoas and they became infested and he lost most of them. They were growing up the back of the tank. He was able to dip the ones on rocks but enough of them hid well in the ones on the back of the tank, so they just kept coming back. He ended up tearing down the tank, and that was a major bummer.
I've seen the leather eating kind take down huge Sarcophytons too - they gnaw their way inside the column and eat it from the inside out, and it goes undetected until the cap collapses. Sometimes it can be saved by lopping off the cap, but it takes time to regrow the column, so it kind of ruins the appearance of what was a huge specimen.
Jenn