Creat a Mask from mndwi index

I would like to construct a mask of the values the mndwi index detected as water in red colour in the image bellow and set such values in red colour corresponding the river water to null or nan specially for Band 3 to be further saved as GeoTIFF, how can I do that procedure?
I think all values corresponding to water (red colour) are above zero.

Thanks a lot.

Two examples, one simple and some more advanced ones, on the use of the Mask Manager are given in this tutorial:
Synergetic use of S1 (SAR) and S2 (optical) data and use of analysis tools

Basically, you enter an expression
mndwi > 0
which creates a binary mask. You can then export this mask as a GeoTiff.

Thanks a lot!!