Insufficient refrigerant ! There must be a slow leakage somewhere.
As usual, there're so many EXPERTS here.
You can only hope the leakage is not with the evaporator.
The reason why ice forming outside is the system cannot keep the high pressure due to insufficient refrigerant. The refrigerant vaporize right after the compressor pipes before entering the house. This is a very common failure of an AC system.
The line from the outside condenser (after compressor) to the evaporator should be filled with compressed refrigerant, in liquid form. The refrigerant vaporize in the evaporator in order to take the heat away, ie cooling air.