I've seen the same on a couple fish I lost. I assumed it's because they had been dead for some time before I noticed them. I'll be following to see others opinions. I can say the couple times I saw this, my other fish and aquarium were not affected. Sorry that's all I got.
All I can say is that it was the first time I saw them, and that when I fished them out all structural integrity of their bodies went away and they became stuck to the net like sludge... Idk... So confusing!