discolored neutral bus screws

I tried to show in the photo below where I have about 4 screws (red lines point to them) on my neutral bus that have a whitish residue on the tops of the screwheads. They are not as shiny as the other screws. Kind of reminds of me of the metal loosing it shiny plateing. I assume this is caused by moisture/humidity which I have hopefully addressed by sealing the conduit into the panel. The discolorization is not on the bus itself and its not on the threads of the screws or on the wires themselves... just the screw heads. Also 2 of the 4 screws that exhibit this discolorization, do not have wires terminated in them. Is this something I should be concerned about? Attachment 39056 (http://www.doityourself.com/forum/attachments/electrical-ac-dc/39056-discolored-neutral-bus-screws-444.jpg)