Icing can be caused by low refrigerant, blocked airflow, problem with cooling water or just setting the thermostat too low for the current conditions. Icing can lead to compressor damage so you need to find out what is going on.
A clogged water pump will cause a high head pressure situation, you'll loose efficiency and capacity (if it doesn't trip first). The icing is from low airflow or a low charge. Are the evaporator coils clean? If you don't run a filter all the junk in the air will settle on the coils and clog the airflow.
