In a 12g tank, and healthy xenia, I think you're in a never ending battle. Anything removed from a rock with the smallest piece of jelly remanining will be a full blown stalk again. IMO, you need to figure out a way to remove it from the tank totally.
I'm even concerned about trimming it and ther mucos it'll create in such a small tank.
Pruning doesn't help these from spreading. Growing is one thing. Spreading is another. Just not enuf real estate in a 12g tank for spreading.
so after you trim them they release mucus? is that bad for your water quality?