Yes you can keep your mac and cheese warm in the crock pot.
But to keep the mac and cheese from sticking to the crock pot while keeping it warm you should spray the crock pot bottom with some non stick cooking spray to keep the mac and cheese noodles from sticking to the crock pot.
If you don't spray the crock pot with some non stick cooking spray the mac and cheese noodles will likely stick to the crock pot.
I've kept mac and cheese warm using a crock pot and just as long as you spray the crock pot with the non stick spray it will work just fine.