I've dipped gonipora, millipora, zoas, palys, acros, and acans in a 50/50 for 5 minutes, with no long term effects on coral health. (been 3 months since first dip anyway) . What I will say is corals don't react well to the treatment, and I can't recommend repeat treatmen s.
I wouldnt write off your coral just yet, it still may recover. It doesn't look like it's dissolving, just like very receded.