How do shotcut masks work?

[shotcut]

Mask = Alpha Channel

Mask: Chroma Key Video Filter

The video filters beginning with “Mask:” let you select a portion of the image or video in which to apply other video filters. There are a few different ones, and this one lets you choose the portion of the image by a color and a distance from the chosen color. [image] See the “Tip” above. To really make this work you add additional video filters that affect the selected color range followed by the Mask: Apply video filter, which combines every thing in between. So, the order of the filters is…

Since you mention “below” maybe you only need a Chroma Key filter and not Mask.