I wanted to update this because I've recently had a setback and then a recovery and it's been pretty cool to watch.
I first noticed about 2 weeks ago that one of the septa was losing tissue on the corner, and over the next couple of days the coral began to rapidly lose tissue. I decided to move it, and in the course of doing so I discovered that it had sunk into the sand slightly due to the current where I had placed it. The tissue underneath had completely receded and the recession that I had noticed on top was this spreading. I'm sure it would have continued if I hadn't moved this out of the sand. I have since set it up on some rock pieces where it can get flow under it as well as above.
At the time I moved it, I also noticed what appeared to be mesenterial filaments on the "inner" face of the coral. I couldn't distinguish a mouth, but I stuck some small bits of food to them the night after moving it and it held onto them for quite a long time. Over the course of a week, these have developed into two mouths that readily accept small food particles, and the coral has rapidly begun to regrow the exposed skeleton. I ended up with three septa exposed, and I hope it will regrow them, however currently it's simply growing down to recover the bottom portion that was damaged in the sand. I've been feeding it every few days, and even though it can't accept very large food items I feel this is helping.
Tough little guy, and I love the color!