I had xenia with my live rock from the previous owner in my tank on day 1. They survived the initial startup of my tank and everything. . (all the corals did) but the rock was from a 2 year old tank.. So, anyways, It doesn't matter what I dod, the xenia are everywhere... LOL. I pruned them just a week ago. Completely removing 1 entire colony of them from one of my rocks. In that week, 2 or 3 heads are in two different sand bed locations. The spot I removed them from my rock has a portion of the colony growing back, and the one colony I left continues to expand.
This all in a 5 week period!!! Hate to say it, but, I don't think weeds grow and spread that fast!!!