Heat Recovery Ventilator - Iced Up

Greetings.  This is my first post/question - many more to follow, I'm sure. 

I live in a rental house that has radiant floor heat and a heat recovery ventilator that pulls warm, moist air out of the baths, kitchen, and laundry room and blows outdoor air warmed through the heat exchanger into the living spaces of the house. 

The  fan started making a high pitched noise this week, so I called and they sent out a service guy (plumber).  The air exchange unit is located in the attic against a south facing wall.  The attic is not heated, but the attic is included within the building envelope (insulation against the roof). 

The plumber, after a bit of effort, pulled out the central core of the exchanger - it looks like a pentagon or hexagon prism with zig-zag filter around the perimeter (not the fan or fan motor).  It was frozen - obviously moisture had gotten in and then the cold make up air froze it.  It's been well under freezing here for several weeks.  The plumber said the ice was restricting the airflow causing the fan motor to work harder.

The plumber changed two small rectangular air filters where the ductwork enters the HRV, put everything back together, turned the fan on, and the sound was gone.  He said the system would be fine. After he left, I checked the air blowing into the house, and it felt like air conditioning - which I really don't need in the dead of winter.

My concern:  When the ice melts, and the water sits within the HRV, will I have a mold problem?

The system looks like it has a light to show that it is frosted up and might auto shut off, but obviously that didn't work. 

Thoughts?  Should I pull the ice core out and let it thaw and dry and reinsert it?  I can live without the air circulation for a couple of days.

Many thanks.