(If they're the small feathery brown ones) Yeah, they're the plague so tackle the problem as early as you can. If you just miss a couple, theyll grow back in no time. I found that it was hard to scrape in all the rocks crevices. I ended up taking all the rocks out that they were growing on. And I covered all the exposed rocks with corals on them with superglue. It took a lot of superglue and a lot of diligence but I killed every last one of them.