Star is in a pose that suggests victory or triumph but everyone else in the pile of goo she is standing on seems to be in some state of horror or desperation... except the eagle, who gives no fucks.
Moon is reaching toward Star in what may be interpreted as a plea for help. Another interpretation may be that she is looking at something horrific and hoping to maintain some kind of control over Star who is suddenly and clearly more powerful than Moon. Another interpretation may be informed by comparing her to Marco who seems to be looking at his hands in an expression of hopeless loss. It may be that Moon is saying, "What have I done?"
However, I think it most likely that Marco, ever the orderly clean freak, is looking at the goop dripping from his palms in horror as he contemplates how hopelessly soiled his hands and body are. Moon, again by my thinking, is likely reaching out both to comfort Star and to seek her help.
Mr. Moon, spider, gnome-dude, buff frog and Ludo seem simply overwhelmed by Stars triumphant goop.
The last that intrests me is Gloserik, whos position and attitude are significant. Where Moon is on the right, slightly in front of the visable edge of the pile, Glossy is on the left and slightly behind. They seem to mirror one another exactly, perhaps indicating that both have are working against one another. They being the only 2 characters focusing on Star further indicates that Star is likely meant to be the tool through which their mutuality exclusive plans are brought about.
The inclusing of the "missing half" emblem in the goo may indicate that the goo is the work of the one controlling that fragment (Toffey) but I think it is intended as a metaphors for Stars failures. I think that the goo represents all of the mistakes Star has been making and how they threaten to consume and destroy the others. Her stance suggests that she will either overcome her faults or remain blissfully unaware of them even while they drown all she loves.