This is a simple ViewShader Fuse to check alpha channels, especially when keying.
This view shader for Fusion displays all areas where the alpha is neither 100% transparent nor 100% opaque as grey, allowing you to easily see slightly transparent alphas. The ones that are hard to spot when keying.
On Windows, copy it to
%appdata%\Blackmagic Design\Fusion\Fuses\ViewShaders and restart Fusion.