Maybe you can share the .skp file so we can see what you’ve got exactly. If the background pixels are transparent in the ivy png you should be able to see through it. Not e that you cannot apply to textures to the same face. Depending on the exact situation it might make more sense to combine the ivy texture with the brick texture into a single image.
What version of SketchUp are you using? Are you still using Sierra as the operating system? Please complete and update your forum profile.