The screws are supposed to be stainless steel however, sometimes something goes wrong with the ~plating of them and they are vulnerable to salinated water and start to rust. Ascorbic acid will remove the stain from the faceplate - you should replace the screws with better quality stainless.
If what you're talking about is waterline 'scum' a paste of baking soda on a 'scrubbie' sponge should do the trick.