Common Name: Also known as the shimmies.  This condition can be caused by a number of external parasites.

Pathogen/Cause: Gill flukes (genus Dactylogyrus) or skin flukes (genus Gyrodactylus).   These are small parasites with long cylindrical bodies and heads with a set of pseudopods on one end and a mouth with teeth on the other.  They are visible under a microscope at about 100x.  To take a sample, net your fish and gently take a swab either the gill plate or the irrated section on their body and put the mucous on a slide.  Under the microscope you should be able to see the little critters.  These animals (monogenean trematodes) are almost always present in your tank.  It's typically when your fish is stressed when they can multiply and cause your fish acute problems.

Physical Signs: The fish's behavior will be the first thing that will alert you to an infestation of these parasites.  However, you will sometimes also notice that the fish's gills become pink and covered in excess mucous.

Behavioral Signs: Fish with dart erratically around the tank, flashing and scratching on rocks and other decorations.  Sometimes they will shake (it typically looks more like vibrate) from side to side, shaking their heads and bodies without moving forward (thus the nickname shimmies).

Potential Treatment:  There are a lot of harsh anti-parasitic medicines on the market that you can choose, however using a combination of salt and a medicine that uses copper as it's active ingredient should work just as well.  You can add up to 1/2 tbs of Doc Wellfish's Aquarium salt per gallon (make sure to add this slowly so you don't shock your fish) and treat with either Coppersafe, Had A Snail, or similar.  Make sure that you treat for at least 5 - 7 days as the lifecycle of the parasites can vary from between 2 - 5 days depending on the water temperature in your tank.

Here's a couple of links for more information about gill and skin flukes.
FishDoc
Aquarium Pharmaceuticals