Thank you for your answers!

For the moment I quarantined it and monitor it closely. I have a plant based fungicide which I plan using, if I see future signs of spreading on its leaves. If it spreads, would it be a good idea to add a calcium supplement at watering?
Indeed it might have been living in an environment which enabled pathogens to thrive. I've noticed white mold on top of the media. But could also be something else. Guess I'll have to wait and see.