I've grown lots of xenia. I had a 180g 8'x4'x1' frag tank with 2 walls of xenia. The tank popped a corner seam and there was so much xenia along the seam that the xenia got sucked into the gap and stopped the tank from draining -- I love xenia.
Xenia will grow across the overflow and require cutting/trimming back. The issue with xenia is that every lil bit can spread. And when you 'pull' it off it tears and shreds.
Clavularia is a better choice for a wall filler like this. Xenia grows in fleshy clumps and colonies that keep strectching out and spreading.
Clavularia grows like skin spreading across the back wall. When it goes to far, or into your overflow teeth, you can put your fingernail under the edge and peel it back like a skin or like a fruit roll up. This makes it very easy to peel it back to where you want it to be and then tear or cut a clean edge.
Dave B