my porcupine puffer is dead - why?

he was doing well. Eating mostly his krill. I did seperate him from the emperor angel because the emperor looked a little stressed. Came home and the puffer was on the bottom and eyes clouded breathing shallow. He was in with a brain coral - a frogspawn - and a giant clam which he left alone. What are the chances that the large brain stung him? His back was scratched up, like pieces of him were falling off in little parts like a shedding colt coral, his skin looked like it was falling off in one spot. Here is the brain pict.

I dont think brain corals are able to sting and kill fish, I think they sting corals as a way of engaging in chemical warfare.

In order to try and determine why your fish died, we'll need more specific information such as
temp
salinity
ammonia
nitrite
nitrate

As well as other infomration
How long have you had the puffer?
Was it eating well, and if it was what were you feeding it?
Other tank mates?
Tank mates that have been added recently?

It seems he may have injury himself. Is it possible there was a rock shift?