Sick/dead coral

[SebastianM]

Hi, quick intro, I’m new to reefing, about 2 months into my nano 10 gallon, everything is going smooth, stable parameters and such. I have purchased a few frags of LPS from a local hobbiest a couple of weeks ago and he threw this Favite (I believe that’s what it’s called) in for free. A few weeks go by and I haven’t seen much activity from this piece, even when feeding etc. The coloration seems okay. Recently I noticed one of the polyps starting to “peel” and a shade of bleach/white forming. I pulled it out to dip in coral RX as I had some mixed up for other frags and noticed the peeling polyp was actually hollowed out, and the piece itself stunk real bad, almost sulfuric. Definitely doesn’t look right, could it be a sting from a neighboring 5 polyp Favia colony, 4 inches away? Or a bacterial infection?