Pycnogonid / spider
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 by Brandon Templin.
Additional information and an excellent picture here: http://www.3reef.com/threads/sps-nightmare-pycnogonids.155311