Mainly, it is unwanted Pube like hairs that grow on the back of the neck, sideburns, or behind the ears, especially when someone hasn't gotten a haircut or a line up in a while. Can also be used to describe any other nasty hairs or substances that are unwanted or "gross" looking.
Crust can also be applied to women.
Someone can be "crusty" if they have these features
With this crust on the back of my neck, i look like a hobo.
Somebody has got to trim this crust from around my ears and on the back of my neck.
Im never going to get any pussy with all of this crust I have on my head.
"Hey man..I need a favor"...."Ok, well ill only do it if you trim my crust first"