Why does my hair straightener have red marks on them and how do I get rid of them?

Everytime i use it this dark red scum like stuff shows up. It must be from the thermal spray I put in my hair first but how do I get it off ? It comes off if you scratch it but it takes a while, isn't there an easier way to get rid of them?Why does my hair straightener have red marks on them and how do I get rid of them?
its all the dirt, oil, and dead skin from your hair burnt onto the iron, just was your hair before you straighten it