Brush on layer mask is inversed

Hi,

I'm working on a project where I need to add a haze, and am doing so by inversing my mask so I can add to the picture. But when doing this, I've found out that when masking regulary, it takes some of the colour out like normal, but when inversed, it instead subtracts what I've already taken out instead of adding more.

This may sound confusing, but I'd really appreciate some help.