Xenia deterioration

Hey everyone! I was just curious if anyone has had an issue with their xenias just randomly rotting/ falling apart? I noticed last night that there was a really pale spot and it looked like it was rotting in the spot. It was about 3/4 of the way to the top. I took it out and iodine dipped it for 20 minutes to see if it would help. This morning I woke up to it spreading to another stalk (I had 3 total). This morning i cut the 2 infected stalks off the rock and put the 3rd back in. Not sure what's gonna happen. It's turned like a gray color and is shedding. I tested everything last night I thought may hurt it and everything is good. Nitrates are low around 0. I know xenias like dirty tanks. Maybe that's why. I've had the same 3 stalks for about 8 months and they were always healthy but not spreading. I also noticed a urchin spine right below the first rotted one. Maybe it stabbed it? I'm not heartbroken over it. Everything else is happy. Idk