mostly that is sooty mold caused by the sugary excretions of things like mealy bugs & scale insect, both of which are harvested by ants who use the extract as food, so inspect the plant for those thript type bugs, and then look for ants any ants they may not always be seen at the time you are looking.

if so you need to move the ants on (not easy in a potted plant), then use a white oil at about 25% the recommended rate. there is a homemade recipe on our site.

len