Are you using the Distance Shader output as the Mask input of your cloud Density Fractal, or as the Density Shader input itself? The latter won't work, it'll just give you all gray. What you want to do is mask the existing cloud fractal Density Shader. I do think a Density Shader is an easy way to do this using that approach. A Simple Shape Shader with Invert on the Mask Input should also work, I think.
- Oshyan