If you let them grow on the glass you can easily peel them off and glue them to a new rock. You can also surround them with small rocks you are willing to remove, and cut the mat with a razor blade after they have grown onto the new rocks and give them to someone else.
I don't know any way to stop them from growing, but I have never tried. I have sold off about 10X as much as I have now, and I have about 5X what I started with. Don't let them spread to something you don't want them on.