If you look at the colour scale of cloud base in Settings, you see that grey corresponds to cloud base higher than 8000m OR that there is no cloud. To check that, just compare the Cloud base layer and the Cloud layer. Where is no cloud, there is no cloud base.
Then, why does this layer look patchy with geometrical shapes? This is because there is no interpolation between each grid point of the model. It gives the same type of image as the Precipitation type layer. The image is not smoothed to avoid mixing of the different levels .
Some additional information: