Now known as Aquilonastra there are 28 species. Almost all are entirely harmless but do reproduce quickly, there are a couple species that will harm corals or other invertebrates but we rarely see them in the hobby. They have a bad reputation because this hobby has a tendency to spread misinformation without further evidence very quickly. So, a few people finding the kinds that are harmful quickly turns into "every one of these eats corals and must be eradicated".