They don't really "take over coral," but they can become a nuisance if they multiply out of control. They feed by putting out webs, which are said to irritate corals (Ive never noticed this so much in my own tanks), but mostly are kind of unsightly.
There are a number of posts about little vermetids taking over a tank, but actually, upon looking at your picture, the type you have are bigger than the ones that I think people complain about spreading everywhere. The ones like yours in my tanks really haven't really reproduced noticably. If I'm wrong about that, someone will probably come by to point it out.
In any event, they're probably not really hurting anything, but they're pretty easy to pry off of the rocks if you don't want to look at them.
