It's pretty tough for them to spread over a sand bed. In my experience even if they do they often eventually separate from the rock entirely and blow around. As mermaid says they also are very easy to trim if they do spread across sand.
I’ve never had any leave my island, 18 different types. Although I have heard about it, I’ve never seen it. Just my experience. Can’t wait to to see your scape in your tank with water!