Some frost will form on the coil around the freezing point. The higher the humidity the worse it gets. It should go into a defrost cycle. Like was said in another thread it's the nature of the beast. Is it not warming the home or are the heat strips coming on a lot?
HP pull heat from the air so when you get to around 45º and less your pulling heat out of that air. The exiting temp on the HP is colder than the entering or ambient temp.
Are all obstructions out of the way of the unit? Are trees above it? is it in the shade? are bushes close to it?
my hp does not run very often because I burn wood. sometimes in the morning the hp will run I have the temp set at 68 . when it does come on the heat is not very warm and does run alot. I hope this makes sense to you
When HP's first come on the air is usually cooler. It takes a little while to actually get up to operating temps. Mine seems to take around 5 minutes I have heard as high as 15 minutes. You would rather it run longer than have it run for a shorter period. If you feel real warm air coming out of the vents then that's your backup heat. If electric you would notice real fast on your power bills. The fact that it seems to run longer probably means your backup isn't even coming on.
If your power bills are acceptable and your comfortable then most likely everything is working as it should. Just make sure you change your filters and have preventative maintenance done on it.
If you recently had a day or two with some freezing rain, or it developed as a result of something like that, you should probably turn it off for a day (switch to emergency heat) and let the ice thaw. With that ice, you are not getting all the performance anyway. Clear it by leaving it off and then see how it goes from there. If it returns, then you might want to have it looked at.