Exterior of the fish is always a little slimy, if it's whole fish. Traditionally, smell it, check the eyes(they should be vibrant and lively), and check around the gills- everything should be nice and red. Here's a couple of questions:
Did you check the pack date when you bought it or just the experation?
How long was it in your fridge, in the packaging?
Where did you get it?
Why was it in packaging? I'm in Seattle and there's salmon everywhere, so maybe that's just me.
Was it previously frozen?
Fish that's off will SMELL