My heating unit outside will not shut off. It looks like the coils inside the unit outside are froze up. I tried to turn the whole unit off inside by first turning it to 50 degrees. I couldn't turn it off that way so I switched it to OFF. It still stays on. I c alled my landlord and they said it was froze up and to leave it on and it would unthaw. Is this really safe?

As far a safe is concerned it is not going to harm you if that is what you mean. If it does not take care of itself as the landlord said by the end of the day them I would be calling the landlord back.

If the outdoor unit is heavily iced, the normal defrost cycle(s) may not be sufficient to de-ice the unit. I would suspect refrigerant loss, or a stuck contactor. Keep landlord informed of developments & allow them to address problem.