One of the blights of many SPS keepers has been the notorious red bugs, which aren't even red! These tiny fleas are yellow and tend to congregate on smooth Acropora colonies. They are tough to kill, using a prescription product called Interceptor from your local vet. Please refer to Reef Central as well as this link for instructions on dosing this product, as it is quite an endeavor that has to be repeated three times and which may affect pods and crabs in your tank. Leaving them unchecked, they reportedly will kill SPS corals one by one. NOT reef-safe!
Update 8/31/06: New page on my site showing how I treated a coral.
Photo by Gregory