What type of mold is it? If it's a two part, and the cracks are somewhere that won't affect the mold locking together you can moisten the plaster with water, and make a small batch of plaster and using a small tool like a stick, just kinda dot the plaster in to fill in the cracks, and then swipe the surface to make it even. It's not a perfect method, but I've used it to fix little issues before.