Mask shader on Android devices

Hi,

I'm currently working on a 2D game. For some feedback purpose I want to use a mask on a quad in order to make some parts of the quad transparents. To achieve this I use two shaders (one for the quad in background, and one on another quad to use it as mask). Here they are:

The shader used on the background quad

 Shader "FeedbackBG" {
     Properties {
         _MainTex("Base (RGB)", 2D) = "" {}
     }
     SubShader {
         Tags {"Queue" = "Geometry+10"} 
         Pass {
             Cull Off
             Lighting Off
             ZWrite On
             ZTest LEqual
             SetTexture [_MainTex] {}
         }
     }
     Fallback "Diffuse"
 }

The one used on the mask

 Shader "TexturedMask" {
 
     Properties {
         _Mask ("Culling Mask", 2D) = "white" {}
     }
         
     SubShader {
         Cull Off
            Blend SrcAlpha OneMinusSrcAlpha
          
         Tags {"Queue" = "Geometry+10"} 
          Tags { "Queue" = "Transparent" }
         
         ColorMask 0
         ZWrite On
         ZTest Always
         Alphatest LEqual 0.5        
         
         Pass { 
             SetTexture[_Mask] {}
         }
     }
 }

The combination of both works perfectly when I test it thanks to the webplayer but on Android devices (Samsung Galaxy S2 and Nexus7) only the background is displayed and the mask is not applied. As I'm not really good in making shaders I wonder what I missed?

Any help would be appreciated. Thank you.