I tried a couple of adjustments
- adjusted the formula to adjust green - but either the stars stayed magenta, or I created these artifacts again
- used an inverted star mask to protect everything but the stars - but because the magenta color is mostly in the fainter outer halos of the stars, it didn't work
- used a range mask that just selected the areas where these issues occurred - but I could adjust the mask fine enough
I ended up fixing it with a completely different idea from how we blended the milky way images:
- I stored the image before running Pixelmath to remove the magenta and after.
- I open both images in Photoshop as layers with the processed image on top
- I create a "Reveal All" layer mask on the top layer
- Carefully drawing with black brush over the areas with the cosmetic issues - thus revealing the unprocessed image underneath
And now I could load it again in Pixinsight for final processing.