Mold in pot?

[gabbro]

Hello, I'm new to keeping orchids, I received a phalaenopsis about one year ago as a gift. Recently I decided to repot it, and soon after I started noticing mold inside the medium. I don't mean white mold on the roots which seems to be a common problem, I mean green mold growing on the bark inside the pot. I've also seen a few previously healthy roots soon start to rot. I tried to add more holes in the pot for more ventilation but the mold keeps coming back. I suspect it may be a problem of the medium, because it was given to me by my aunt who had orchids in the past but it's quite old, and the problem also never presented itself before I repotted.
Can anyone give me advice on how to handle this? I'm looking to buy new bark to put in place of the moldy one, but I don't know how to treat the orchid in case the mold got on the roots too. I'm just very confused as to why it's happening, because I'm watering it as often as I did before and it seemed to be alright back then.