When I hear about a new pest, I'm always curious to learn more.  Here's a thread about a spider (Pycnogonid) that actually eats acropora tissue.  

http://www.3reef.com/threads/sps-nightmare-pycnogonids.155311/

This is one creepy little beast.  Apparently visible after lights out, using a flashlight and some heroic vision on your part since they are tiny. As with any pest that is causing damage, the trick is finding the best solution that won't hurt the livestock.  Doing a cursory search, people have been dipping infected corals in various products to kill off the unwanted creatures. Looks like I have a new critter to add to the ID page as well as the Quarantine & Dips page. 

It's best with all new coral arrivals to put them in a quarantine tank to observe them closely for a few days. One person pointed out that you can spot these pycnogonids on pocillopora too.