Can I use a heat gun to melt the Ice from the external part of split ductless system?
DAO1 is right, if this happens often, then your defrost capability are suspect, and you should call a technician.

For the time being, if no one is currently occupying the units, the best thing to do would be to run the system in AC mode.  This is optimal for 2 reasons:
1) It will cause a more gradual change to any of your system parts, meaning there's less of a chance of parts cracking or splitting due rapid expansion. 
2) It will heat the ice from the inside, meaning you shouldn't have to melt the ice all the way though to remove it.

That said, the radiator fins are built for heat transfer and heat alone shouldn't damage them, so a heat gun on setting you could point at your skin for a minute with out pain would likely do the trick without damage.

You may also want to consider using a space heater in the future, especially when the temperature is below 5 degrees F because your ductless heat pump may can have a negative coefficient of performance at that temperature as indicated by Figure 8 in this NEEA study.