This time of year we get a lot of calls because of the light on your heat pump is flashing. If you have just one slow flashing green light, this is just telling you that your outdoor unit is in defrost mode. Another sign is that the fins will be all the way up and it will be gently blowing cold air. This defrost mode usually only last 10-15 minutes. The colder it gets outside, the more often the unit will defrost itself.
There is nothing for you to do, the unit will take care of itself!