When xenia is happy, it is hard to get rid of. I had so much that all my LFS and friends had more than they wanted...
Try a long spine urchin. I had one that would mow 4 square inches a night. I sold him.
Peel it. Not 1 or 2 stalks. All of it... every stalk. If it doesn't peel off easy, or tears. It will come back, and you'll have to do it again later.
Good luck, and congrats on a great tank.
BTW, test for phospates, and other "nutrients". I am willing to bet your tank will test zero on all, but test your top off water, your salt water (what you use to change your water) etc. My tank was very balanced, and my xenia was under control. Then my xenia went nuts... over growing everything, see above comments. about 4 months later, I had so much algea that I was losing corals left and right. Everything out of control.
Then I figured out my source water suddenly had phosphate, alot of it. It took 6 months of water changes, phosban reactors, charcoal, etc. to scrub the tank.
Now, algea is gone, xenia is growing at a normal rate, all is good.
I am not saying this is what you have, but check it out just in case... it bit me hard.
Good luck,