Mine once withered away and turned white. They turned to powder when touched and just disspeared. A week later they were back and CRAZY! They grew all over the dang place and were making new polyps every single day!
Now I am at war with them. I pulled them out and they release toxins into the water. I run carbon, then pull some more. I hate xenias! Ugly brown color and really REALLY long!
I dont know why people like them so much. Maybe in a large tank and them kept surrounded by sand, but in a small tank, they can be a bummer.
I wouldnt worry about them! I will bet you they come back and make you hate them!