Why can't you just edit the Solder masks on EasyEDA editor ?

You can not? Every pad has paste mask, I can only expand it in pad properties, but not remove. Am I missing something and you can remove mask from a pad in EasyEDA?

Yes, you can only edit the expansion, you can't delete it. you can use solid region instead of this pad. multi-layer pad doesn't have the paste mask, or you can change this pad to multi-layer

To clarify, The EasyEDA PCB Editor allows: 1. automatically placed Solder and Paste Mask apertures to be expanded but not deleted; 2. Additional Solder and Paste Mask apertures to be created and edited. Note that: (a) additional Solder Mask apertures can be created independently in any shape including as text and in any location; (b) additional Paste Mask apertures can be created independently in any shape (except as text) and in any location even if they are non-sensical. The screenshots below show the EasyEDA PCB view and the Gerbers of a test PCB with several additional Solder Mask apertures (including one as text) and in the upper right corner, a small rectangler aperture of Solder Paste Mask overlapping a small rectangular aperture of Solder Mask. This means that it is possible (but non-sensical!) to place an aperture in the Solder Paste Mask layer over an area that is covered by Solder Mask (i.e. not over exposed copper). EasyEDA PCB Editor view:  Gerbers in gerbv: 

@UserSupport I'd love to use solid regions, except you can't number them in PCB libraries, so it's physically impossible to create a paste-mask-less footprint.

at v6.2 the mask expansion will support to set nagetive value.