I had a low pH problem in my tank that lead to cyano and dinos. I lost some zoas, and to this day i'm just starting to get the zoas back to healthy.
But I was scared for my Rhizo. It started to receed on the edges and never opened up. I continued to feed it by removing it from the tank in a bowl. I went to daily feedings. Slowely as my tank got turned around, so did the Rhizo. Now it is always open and the edges have grown back out. After this experience i would say they are very, very hardy corals.