A heat pump outdoor unit spends part of its working life covered in frost. That is part of the design. The owner's job is to tell the normal frost cycle from buildup that needs attention, keep the unit able to breathe, and note what you see.
What normal looks like
In heating mode, the outdoor coil picks up frost. The unit then runs a defrost cycle, which melts the frost and may send steam up from the top. During defrost, air from the indoor vents can feel cool for a short time. The unit then returns to heating. Frost that clears on its own is the normal pattern.
What to note
Write down any of these: ice that covers the whole coil and stays after defrost, ice that spreads onto the fan guard or fan blades, a unit that stops running after a defrost, or a loud grinding or rattle during the cycle. Note the outdoor temperature range and what the unit was doing at the time. Those notes are what a service provider will ask for.
Owner routine
Keep the area around the outdoor unit clear of snow, leaves, and stored items. Keep the clearance around the cabinet that the manufacturer's manual calls for. Do not chip ice off the coil with a tool, and do not pour water on it to speed things up. Check the indoor air filter on a monthly schedule and replace it when it is dirty, because restricted airflow can affect the defrost cycle.
When to stop and call
If ice is heavy and the unit is not clearing, turn off power to the outdoor unit at its disconnect and call a service provider. Do not rely on auxiliary heat as a fix for the ice. Tell the provider what you saw and when you saw it.
Plan around the age
A heat pump's age is a planning input. Add the install year to the service life on the heat pump typical service life and when to plan page, and you have a window for the next decision. Use that window to plan, not to worry.