it needs to drier than if you were serving it straight away. I find setting the pan, after draining, until it's finished steaming, and not adding milk, will help, with the potatoes still a bit hard. Otherwise moisture will separate out when it thaws.
some varieties of potato are worse for shedding water.
I freeze it in 'balls' (use a scoop and freeze on a board/baking sheet) then transfer balls to a freezy bag once solid; then either reheat in the oven like you would with say frozen aunt bessy bubble and squeak etc, which gives it a bit of a 'crust' or alternatively microwave and rem ash with a fork. I tend to do potato and swede or onion or something like that to liven it up a bit.
This way you can do as many 'balls' as you like depending on hunger level/number being fed