Cullingmask issue.

I'm having trouble understanding a rather simple operation (I'd think it would be) involving a shader.

What I'd like to do is have two textures -- a planet texture, and a clouds texture, and have the clouds' offset shift gradually to make it look like the clouds are moving over the planet -- but I can't seem to get my masking shader to work correctly. Instead, what I'm getting is a big white blob. I really want to understand what's going on in the shader here, but I feel a bit lost -- what I want is for it to ignore the color in the mask, and only display the color in the texture when the color in the mask is white. I've attached a screenshot of what's going wrong, and what I'd like. I understand that it must have something to do with the Blend mode that the shader is using, but I don't really understand the vernacular on the unity page. Any chance you could explain to me what's going wrong, and how to fix it?

Here's what I've got:

Here's my planet with the clouds, but without the mask. Still kind of yuck.

Here's the shader:

 Shader "MaskedTexture"
 {
    Properties
    {
       _MainTex ("Base (RGB)", 2D) = "white" {}
       _Mask ("Culling Mask", 2D) = "white" {}
       _Cutoff ("Alpha cutoff", Range (0,1)) = 0.1
    }
    SubShader
    {
       Tags {"Queue"="Transparent"}
       Lighting Off
       ZWrite Off
       Blend SrcAlpha OneMinusSrcAlpha
       AlphaTest GEqual [_Cutoff]
       Pass
       {
          SetTexture [_Mask] {combine texture}
          SetTexture [_MainTex] {combine texture, previous}
       }
    }
 }