I'm not totally sure of the cause but I've had that before as well and, as you noticed, it has almost always associated with low alkalinity/low pH (btw, what is your pH at?). If your parameters are back to normal, it may be possible that it is a protozoan or bacteria that is causing it. I have an A. plana that is doing something similar at the moment (but in my case it starts at the base and extends as a well defined streak down a branch, causing the tissue to turn hard and the skeleton to be soft and crumbly). It's not a disease that has been described in peer-reviewed literature yet but I've found mentions of it in various papers and they tend to point toward bacteria or protozoans that actively penetrate and erode the skeleton.
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