Sooty mould by the look of it. No need to panic. I suggest a complete repot, and wash the whole plant, the pot, everything in a disinfectant. You give no personal details which makes it hard to advise properly. - where you live and how you grow enables advice to be better targeted, also affects availability of supplies - what is on sale where you live may be quite different from what I can get, for example.
But if you live in USA or certain other countries ( not in Europe) you should be able to get Physan. Dilute that to 1:1000 for this washing purpose. Then use it for all watering and spraying at say 1:2000. Hygiene is the name of the game.