The correct spelling is "말고요"
말고요 is composed of "말고" + 요(honorific ending suffix)
Some accents in Korea (usually older people who grew up in the countryside somewhere) change the pronunciation of “말고” to “말구.”
A 말고 B means ‘not A but B’, ‘except for A’ or ‘B instead of A’ in English.
You can use this with nouns or verbs.
noun(A) 말고 noun(B)
이거 말고 저거 주세요.
Give me that one, not this one.
우리 커피 말고 차 마셔요.
Let’s drink tea instead of coffee.
그 사람 말고 민수 씨하고 가고 싶어요.
I want to go with Minsu instead of that guy.
저 말고 다른 사람한테 얘기하지 마세요.
Don’t tell anyone but me.
이 색깔 말고 다른 색은 없어요?
Do you have other colors except for this color?
If you use A 말고요, it means "something else instead of ".
빠른 성공말구요 = 빠른 성공말고요
(I mean/wish that)something else instead of fast success.