Scumph is the nasty yellow residue of something. For example the yellow stuff on the cigarette filter after it's been smoked. Or the yellow stuff that comes out of your ear when it hasn't been cleaned in a while. Anything dirty, sticky, yellow and that is a residue of something could basically be called scumph.
"You got some scumph in your ear... man c'mon clean that up"
"I need to get all the scumph off the floor before my parents come back"