Probably nothing, as long as the ice is just a thin layer. In order to understand why the ice is forming, let’s start with the basic principle of how a heat pump works. It does not create heat; instead, it collects and moves it. In the summer, a heat pump absorbs heat from inside the home and moves it outside--thereby lowering the inside temperature. Then, in winter-time, it does the reverse: absorbing heat from outside and moving it inside.
A heat pump air conditioner operates in three modes: cooling, heating, and defrost. When the system is in heating mode, the coils are in what is called “evaporator” status. They get quite cold and, under some combinations of outdoor temperature and humidity, frost will form on the surface and begin to build up. A sensor at the coils recognizes when this occurs and switches the system into defrost mode, which reverses the flow of the refrigerant for just a minute or two to warm the coils and melt the accumulation of frost/ice. The fan at center of the unit shuts off while the defrost is happening. You may see some water vapor/steam rising off the coils too. When the temperature of the coils reaches about 57º F, the defrost cycle is complete and the system returns to heating. A defrost cycle will occur about every half-hour to hour during very cold weather.
