Ooh, vague memories of trying this and it ending badly - the tops went blackish, if I remember correctly. There's always a chance, however, that this was a self inflicted anomaly so perhaps worth a google.
Perhaps if you covered with film and pressed with something heavy so potatoes are submerged?
Or perhaps you could cook it through, leave 20 minutes off the cooking time, so there's no chance of discolouration and less cooking time the next day, perhaps top up with a little more cream or dot with unsalted butter?
I do one from bBC good food and always cook it the day before let it go cold and slice it into portions put it on grease proof paper in the oven for about 20 mins to warm through. It stays together in nice portions too
When I need a potato dish prepped beforehand, I add finely chopped fried onions, garlic and herbs to freshly cooked, mashed potatoes. Put in a baking dish and cover with grated cheese (and more herbs, plus sliced tomatoes if I have any) then put in the fridge once cooled. I much prefer stuff I can just bung in the oven and not worry!