Heat pumps do frost and can have alittle ice build up.
Usally starting around mid to low 40's outside air temp.
It could be a problem with the defrost cycle or it maybe acting as it should.
If your thinking the unit has a problem, call a service company out to have a look,
A heat pump is a have to see it, to know kind of thing.
To many variables with them. And any suggestions would be a guess,