시무룩 means sullen, long face. 꼬물 is the name and 꼬무룩 is long face 꼬물. It's abbreviation. 꼬물+시무룩. If the name is 슈크림 you can call 슈무룩 (long face 슈크림)

It's an onomatopoeia. like a snake or worm makes its body circular. and when someone looks down, we can say to him, why are you so 꼬무룩? for example A: 왜 그렇게 꼬무룩(or 꾸무룩)해있어? 무슨 일있어? (why are you so down, Is there any problem?) B: 그게 있지... (Actually...) thanks!