Hydroids 99% of the time aren't going to hurt anything. They possess zooxanthellae and are filter feeders. Although that is a real good site for identification it is not always correct on the facts. Colonial Hydroids could be a different matter as they may sting desirable corals, etc., that are close to them. Here's a link, so you can come to your own conclusion. Hydroids: Chances Are We Have Them but Do We Want Them in Our Aquariums - Yahoo! Voices - voices.yahoo.com