Xenia are capable of being mobile (probably at a rate of a couple of mm per day if they're not happy at their current position, if their current position is adequate they'll stay where they are). Xenia clones or 'offspring' seem to tend to move away from the 'parent', usually toward higher spots in the tank (probably to get their decent share of the light).
They're also capable of completely detaching and have the water flow carry them to a potentially better place. I'll sometimes find xenia 'orphans' on in an area of the tank completely opposite from the main colony.
They can grow on the sides of the glass and even powerhead cords, so any surface will do. I've got xenia growing on torch coral skeleton.
