Garage Mold, is Landlord Responsible?

[Phil Goodwin]

I have a  building with several garages on the first floor with an apartment upstairs.  Property is located in Philly suburbs.  The garage is old with a sliding barn door that does not seal the inside from the cold weather.  The ceiling in one of the garages is drywall and it is growing mold .  The apartment has a hot water baseboard heater above it but there are no signs of water stains or drips.  The lady upstairs says the heat works fine.  The pipe for the baseboard heat is directly above the area with the mold.  My plumber checked the connections for the radiator in the apartment but he did not think the valves at the radiators were leaking, and the carpet is not wet either.  The guy that rents the garage does not live in the apartment upstairs, he texted me today and was really upset that mold was on his belongings (wood chairs, tools, bedroom furniture, etc).   

I noticed the mold about 2 months ago and sprayed Concrobium on the drywall ceiling, it seemed to kill the mold.  And I put a dehumidifier in the garage to help get rid of any moisture.  Although since the door does not seal the garage I am not sure the dehumidifier did any good.

I have called a local mold remediation company and they are coming out tomorrow to inspect.  I am hoping they can identify the issue.  Maybe the pipe is getting condensation on it since the garage is cold?

The garage tenant is really upset and wants me to pay to clean or replace any damaged belongings.  Am I responsible?