Q: If someone plays Beam Us Up, and one player had a being with a Creeper attached, what happens?

A: The Keeper and Creeper would get sucked up into your hand… but a Creeper can never be in your hand, so the Creeper is then immediately played back to the table by that player. It would either then reattach to some other Keeper if possible, or hang around for as long as it takes for a new Keeper of the right type to show up to attach to.