Thats really odd. I finally managed to control the spread of my xenia.
I use to have 1 stalk multiply into 6 in less than a month.
Now I have 2 stalks that haven't split in over 3 months. I agree with Bev that the flow is the key to these corals. The higher the flow the more it wants to split. My 2 stalks are in a fairly low flow spot and are waving very gently in the current and pulsing away to their hearts content.
I hope you figure out what is wrong with your xenia. And yes a pic would really help us out..
