I got a straight pipe done on my PD170 VRS less than a year ago, the tips are now looking like this. 
 

The guy who fitted the exhaust also supplied the pipe and tips, is it rust and can I remove/ prevent it? 
 

Thanks 

https://imgur.com/a/pm7p26X

You could try some Autosol chrome cleaner or some scrunched up tin foil dipped in water, what I did with mine is changed it to a stainless steel tip and that has been on for three years now and only needed polishing a couple of times.

Or scrunched up aluminium cooking foil dipped in salt water and give that a wee rub.

 

For worse on chrome dip it in cola.  I prefer to drink the cola and just use the salt water and foil. 

 

  Really cleans up old push bikes and the likes chrome.  But that is proper chrome.