Homemade mac and cheese always loses its creaminess and gets dry after sitting out a short while
Honestly, you either eat it fresh, or accept it will congeal.

There are 2 factors going on, 


the pasts will keep absorbing water until there's almost none left
the cheese will set as it cools.


The upside is 'yesterday's mac cheese' is great in &amp; of itself.

Your only 'fix' is make it wetter or eat it sooner.