HELP!! Is my Silver Shark ill?

Hi over the past week my Silver Shark has developed a kind of discolouration to the end of his nose. I had him checked out by my aquatic shop where i got him from and we're all baffled....IT'S NOT : White Spot, He has NO fungus forming, there is NO sort of 'cotton wool' on him and he generally seems fine, He's still eating and swimming fine, but now what was just a slight discoluration on his nose has spread right up his face to around his eyes. He looks like he has got a 'greyish white' nose and a silver body. All my other fish are fine and i have one other Silver Shark and he's ok too, What shall i do? I don't want to waste a vet's time and my money on something that could just be his colour, but i don't want to chance loosing him to some terrible disease eigther!!