I used royal icing transfers for the ghostbusters toppers. Basically I put the print outs under wax paper (I taped it down so there was no chance of movement) and piped with flood icing on top of the picture so that I could get a perfect version.
I waited a few minutes between each color, letting them dry under a fan.
I let them dry over night before I used a Foodoodler to draw on the face and details.
For the slimers I piped the body in sections letting each one dry for a little before adding the next.
The ghostbuster cupcakes were strawberry cake with vanilla icing and the Slimer was vanilla cake filled with lime curd slime and topped with vanilla icing tinted green.
I heard they were a hit!