Masking is important in Photoshop and there are lots of situations when you need to know how to better control the mask, it’s shape and feather. And sometimes you have to know how to adjust only some areas of the mask.
The problem appears in many situations as a white outline around your masked element: this occurs because of light surrounding the element. To eliminate this effect you need to mask it properly but even so, depending on the overlapping background color in your Photoshop scene that white outline will be seen or not.
You can select areas of the mask and use Levels to adjust how much the mask is shifted inside or outside, thus controlling the appearance of the white line. It’s simple and useful!