Ask-a-Dave's-Gardener: Need Help With Rubber Plant, 1 by servanezie

servanezie wrote:
UPDATE:
So the leaf that seems to be infected got worse today. The browning spread further and when I checked on the leaf underside there are some grayish things underneath. They are circular and seem to be wet and oozing out from the leaf. I\'ve attached photos of the leaf today.
I already cut the infected leaf and sprinkled some cinnamon to the cut part as I\'ve read it prevents fungus.
Can you tell me what are those things I underneath the leaf? Is it some sort of fungi or bacteria that I should be worried about?
What should I do to prevent it from happening again?