Black rot or culture? and an ailing phrag

[lkg1323]

So a couple of my catts developed very small black tips on new growths. At first I thought they were sun burns, but they developed into more what looks like black rot. If I let them get bigger than a cm they are wet/mushy (rot). I've cut sections, only for it to come back as in the second photo, which is a dryer rot than previously.
Is this black rot? Is it imperative to remove the whole new growth?

And what causes this? Overwatering and/or lack of airflow?


I also (sigh) am seemingly killing my phrag pearcei. It looked great and it is now kind of sulking. A brown tip transformed into this whole yellow leaf in what seems like overnight.
I've also removed 2 smaller/newer leaves from this guy that appeared yellow/dead. I know it might have been a mistake - I am hoping this could be a Mg problem?