A mustache that is not fully grown in yet, and is still growing, so the hairs are spread apart. Scumstash mostly applies to mexicans who are dirty, but can be applied to noobs with a mustache thats dirty.
Crystal: "Lucas, I love your scumstash, it's so dirty."
Lucas: "Thank you ese."
James: "Yo scumballs, get you and your shitstash outta here."