SageMathCloud is the former name of what is now CoCalc.
All SageMathCloud accounts were turned into CoCalc accounts. Users can access them with the same username and password (or using Facebook, GitHub, Google, or Twitter authentication).
The initial name reflects the goal to make SageMath available online, without having to install anything on one's computer.
The new name emphasizes that CoCalc, more than just a way to use SageMath, offers a whole collaborative calculation environment.