Sorry, I've been trying to get it out of my tank for 3 years now. There really is no thinning it out it will grow back and spread. My only real solution has been to take the complete rock out, scrub it clean then let it dry out for a few months on the porch before cleaning again and putting the rock back in. You can peel them off the rock but they have to be big enough to get a good hold on the stalk. The catch here is if any piece of the Xenia no matter how small gets loose and floats off you will have a new growth where ever it lands in a month. Again, removing the rock is the "hard" but sure option. If your LFS wants it ask if he will replace your rock with a "clean" dry/live rock, all they can do is say no. Good luck!