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When you chill and reheat butter or cheese, the oil separates from the fat and leftovers can get really oily. I've never made a macaroni and cheese or cream sauce that didn't separate after I put it in the fridge. My only solution is to not serve leftovers to guests!

what kind of cheese did you use? check the ingredients on the cheeses... some are not made with milk at all... but with "oil".... that's why some pizzas ooze grease? it's the cheese that's doing that.....

When I make mine I do not do the topping the first bake. If I reheat I do and then it absorbs the cheese sweat. I only add 4 tablespoons to mine and I melt it and stir it in.
http://www.seriouseats.com/talk/2008/12/upscale-mac-n-cheese-any-ideas.html

i would cut back on all the cheese on top. try using equal parts of cheese and panko pulsed together in the food processor.

I recently read that the oil on pizza is from poor quality cheese that contains oil as pooch said. I would thin that using a high quality milk only cheese would help. I would also consider using less butter, just a little less so that you don't have to add any other ingredients and alter the recipe.

Try the Cook's Illustrated recipe. I made it last weekend and the leftovers have been amazing. They recommend a combo of cheddar and monterey jack for optimum flavor and minimum oiliness.

I use a Cooks Illustrated recipe as well and love it; a word of caution....CI has many Mac and Cheese recipes, some very similar, some very different.....apparently even The Great Ones can not agree on "The Best" recipe!
(I use the one from the Cover and Bake Cooks Illustrated Book, but I alter it just a little to suit my tastes. shhhh, don't tell! I wouldn't want them to cancel my subscription!) (I tease, I am a fan...)

I've never ever had my mac n cheese become oily when reheating. Actually mac n cheese should be better as leftovers.

My only guess is that you picked up some oil based cheese rather than whole milk.

Interesting. I was always told that putting direct heat to buttermilk will cause it to break and curdle, but maybe that's a bonus for mac and cheese, etc.

I've been reheating it in the microwave. And I checked the cheese, they were milk-based - I used extra sharp white cheddar, ~ 6 oz gruyere, and ~2 oz pecorino romano. Thanks for the suggestions - I like the idea of not using the topping on the first bake... maybe what I will do is put topping on part of it for the first bake (on the part that will get eaten that day), because the topping is so good! I will also try to change up the cheeses and try the cheddar/jack combo.

Did you taste the cheese sauce with your cooking spoon before adding it to the macaroni? A littel known fact, and the reason most chefs carry a tasting spoon in their pocket, is that the Carbohydrate cutting enzymes in saliva do not get destroyed in cooking, so even the slightest amount introduced will cut corn, wheat and other starches when held for long times at serving temperature, or refrigerated. Stirring it after tasting introduces enough of the enzyme to break the sauce on reheating. THis may not be the reason, but a nice piece of information, because so many family cooks taste as they prepare and later wonder what went wrong with the leftovers.

I used the Martha Stewart recipe over Christmas and it came out great. I ended up switching cheeses but stuck with the same amount the recipe called for. It makes a huge amount so I had pleny of leftovers which I warmed up in both a microwave at work and in my oven at home. In the microwave I stirred it about every 30 seconds and at home I placed it in a cold oven and then turned the heat on (300-350) and let it warm up slowly.

Reaheating mac and cheese is a problem, but there are solutions. The first is not to use a bechamel - the flour/butter contributes to the grease that's already in the cheese in the dish and the fat separates out on reheating, and IMHO, it adds a pasty taste anyway. I use ricotta cheese instead of the flour/butter.

Reheat it on in a microwave or on the stovetop, stirring a few times while it's cooking; also stir a few times to distribute the remaining crust into the dish before you pop it in. You might have to add a little more milk, as also suggested above, to restore the consistency. It won't be exactly the same as when you first removed it from the oven, but it won't be drowning in a puddle of fat.