If you see frost building on the indoor unit, the copper line set or the condenser.
This usually relates to 2 potential issues.
1.) Your mini-split can have a restriction in the pressure, which means that there can be blockage in the lines due to the way it was vacuumed or your expansion valve might not be opening fully. You will simply have to verify the pressure on the low side and if you notice the pressure slowly or quickly dropping down to almost 0 psi, that confirms your EXV valve being restricted. Before replacing the valve, you can try to slightly tap on it with a ranch to see if the pressure will fluctuate or verify if it's not dirty or rusted.
2.) Second issue would be that your unit is low on refrigerant. If the pressure on low side remains low but does not drop, this usually indicates your unit being low on refrigerant.
In which case you would simply have to contact and HVAC technician to verify the pressure on the line set and to verify there isn't a leak in your system which can also cause frost to build up on the line set and unit.