What Causes White Crust On Nipples While Breastfeeding?

I am currently breastfeeding- pumping and noticed (with my first child too) a white crust, almost like dried milk on the tips of my nipples. It doesn't easily come off--I have to exfoliate them to clear away the little bits of stuff. What is this, and is it normal? Should I continue to exfoliate my nipples after I pump?