i never had any negative problems with my corals, so i noticed them way before they had done any damage.
i noticed them mostly on the undersides of branches away from the bright light. take the frag and put it up close to the glass (still under water), and rotate the coral to get a look at the undersides of branches that dont get much light (the color of the coral skin will look lighter colored due to lack of light), they usually hide there the most IME. be sure to turn the pumps off when you do it.