@Nav_Varda This FAQ article might help for that What is the meaning of the layers in pcb_new and in the footprint editor? (KiCad 5 and earlier)
And possibly also How does solder mask layer work? (Note this focuses on the normal workflow of letting pads define the mask layer. It might however at least partly apply to your usecase as well)
Be especially aware that you really need to check the gerbers not the 3d viewer! There is postprocessing done on the mask layer on gerber export. Most importantly the minimum mask width (minimum web) is applied only on gerber export. (This will mean that some of your thin “green” slivers might vanish)