I have those starfish all over my tank and they do not cause anything any harm. I have acros, m. caps, candy canes, zoanthids, and a kenya tree in my tank. I have seen them crawling on all and damaging none. They seem to be just after the algae. As some of the articles say above, though, there are varieties that do cause harm. I think those varieties they have more of the the purple mottling on them (from what I've seen of the bad pics -- my experience is limitted).
The blow-torch method is only temporary in dealing with hydriods. They always come back in a couple of months unless I put some zoanthids over the patches I clear. Then again, I have a bad case ("Those are just brown tube worms. Honest"). They are the dominant lifeform in my tank due to inheriting "used" live rock that was severely infected and then feeding them with high nutrients and live phytoplankton for a couple of years.
If I could get starfish that eat hydriods in addition to SPS, I'd happily dump them in my tank, though.