When stretching NB images you'll get a far more pleasing result if you don't stretch everything as far as you can, like RGB. Try stretching to balance the histograms.
Pick the weakest channel and stretch that out, normally SII, then stretch the other 2 channels to match.
Also have a read of this page:-
http://bf-astro.com/hubbleP.htm
This is your image with the green knocked back a bit.