What happens is the Xenia starts to "divide and multiply". Basically a stalk will grow another stalk and this new stalk will start branching across the rock it is on. So if this "one lone rock" is near other live rock, it can branch off onto it.
If this one rock is very far apart from everything else, that would be best.
The problem though (and I am going by memory so I may be off on a few things, it was about 2 years ago I had the problem) is that Xenia act much in the same way as Aiptasia, in that if a piece of it breaks off or gets cut off, wherever it lands... tada, new Xenia.
In my case, I believe I had either an Emerald Crab or something else in the tank that would pick at my one Xenia colony, which began spreading Xenia spores all over my tank. In a matter of months, it was out of control.