There’s some great suggestions here, if feasible - but if it’s just round the windows, I’d probably just wipe it off and not worry too much. I don’t think I’ve ever lived anywhere where that didn’t happen in at least one room.
Some “mould and mildew spray” from your local “bits and bobs” shop would do the job fine, or sugar soap if it’s really bad. Any general cleaning spray will get it off with a bit of effort.
This only applies if it’s just round the windows and windowsills though - watch out for black mould on/behind the wallpaper, in the walls, in the paint, back of the wardrobe, back of the bed, back of drawers etc - that’s potentially a whole different level of grim. Try and keep your house above 10°C, even at night, that stuff really spreads when the house gets cold.