The Pycnogonid is a sea spider that apparently preys on SPS corals like Acropora and Pocillopora. These are very hard to spot, and require a keen eye to even notice their presence. Hobbyists wait until lights out and use a flashlight to spot this pest. No known predator at this time, they are removed by dipping the coral in a solution like Bayer, CoralRx, or similar.
Photo above by Brandon Templin.
When I hear about a new pest, I'm always curious to learn more. Here's a thread about this spider (Pycnogonid) that actually eats acropora tissue. http://www.3reef.com/threads/sps-nightmare-pycnogonids.155311
This image is from Rhoads238 in that linked thread.
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.