To keep mac and cheese creamy when reheated you need to add some more milk to the mac and cheese before reheating it.
The secret to keeping your mac and cheese creamy and make it come out just as good when reheated is to add some more milk to the mac and cheese before you heat it back up.
When you add the milk to the mac and cheese it helps it retain it's creaminess.
Just like when you make mac and cheese you add milk which gives it the creamy texture and creaminess.
You can make mac and cheese without milk and just use water but the addition of milk makes the mac and cheese much better.
It's best to put the mac and cheese in a microwave safe bowl and add the milk and then cover the bowl with some plastic wrap and then microwave it for about 30 seconds to a minute.