Yeah, be careful. That particular family either requesters nematocysts or stores toxins, so, don't handle one. I can't help but think yours was just looking for its food source, but its definitely possible it was the culprit. Like Bang said, they usually loiter at their food source for very long periods, or search for their food source if they can't find it, so if it were munching there, I think you would have noticed it before.