Alpha Channel Issues

Hi,

I am fairly new at this, however I am using Fluid mask which works great, however when I try to make an alpha channel to use as a layer mask I get a slight fringe around the edge which looks bad when replacing with a dark background.

The png or layered tif that fluid mask spits out looks great, but when I use this to make a selection it shows the slight edge.

Any suggestions, happy to try a new masking software.

Please help as its frustrating me!

Regards

Samantha