Does it look like this?
http://www.fishbase.org/Diseases/Dis...D1.jpg¬com=
If so you are in for a bit of fun. This is usually refered to as Black Ich and is a sign of parasitic worms being present. It's pretty easy to knock the worms off the fish using fw dips, formalin or keeping them in hyposlinity QT tank for a bit.
Getting them out of your display is going to be a bugger though. If you don't have any corals I'd move the shrimp out and do a week or two hyposalinity in the display, that should get them.
Here's a pretty good synopsis for you:
http://www.petsforum.com/personal/tr.../blackich.html