I think it depends on the species, but there are tons of species that look similar. I think they ate my space monsters, others think they just melted, others claim that they've never ate their zoas, who knows?
Ive never seen them eat corals but there is some discussion on the board about whether they are or not. My old tank had them and they did a meager job of algae eating. Im planning on adding some to my DT tank and then culturing some in a smaller tank, maybe a 10gallon using some nori.
EDIT:
Looks like I should consider the species, but if I'm feeling iffy about it, ill just keep them in the culture tank.
I also have a bunch and do not consider them pests. They have never touched any of my corals. I do second that they do eat coralline alage but most the time they are on the glass cleaning it lol.