How to remove white marks from PLA
As suggested by Ryan Carlyle you will need to use heat to reflow the plastic. However I recommend against using a heat gun as generally you want a very concentrated small area to be heated.
I have had great success in using a soldering iron with the tip placed close to (but obviously not touching) the problem area. Keeping within about 3-5mm has very good results, is easier to control and position. This limits the impact to the surrounding areas and can let you get into small crevices etc.
Just be careful not to touch the part.