Ice usually form when suction gas pressure has dropped to ~ 40psi to 45psi (on a r22 system), ~ 80psi on a r410 system. As such, it could mean that your system has a problem of gas leak.

However, if you see the entire fan coil turning into a block of ice, it is most likely caused by a faulty magnetic switch that doesn’t cut out when you turn off your indoor air conditioner; meaning, the outdoor condenser is still running and continuously supply the refrigerant to your indoor aircon blower. Modern inverter system do not have such a problem, they do not use magnetic switches to cut in the compressor anymore.

An extremely choked fan coil can cause ice forming on the fan coil. When the fan coil is choked, 2 things happens: First, the liquefied gas has difficulty fully evaporating and that will cause the suction pressure to drop by 5-10psi. Secondly, when the fan coil is choked, the fan coil is in fact colder than normal as the coldness is not dissipated well enough.