Why is my mac 'n' cheese grainy?
The problem is your putting all your cheese in at once. These cheese is going to settle on the bottom which would cause to clump up and be gloopy.

you need to stir small portions of the cheese at a time until it is melted. rinse, repeat and continue.

of course this is a crock pot recipe....

your probably best to make the cheese sauce portion on the stove first, then combine in a crock pot after.