First as someone said already - find a water source. If you don’t find the source then no matter what you do you will get this shit again, but worse.

Second, just scrub that abomination away ASAP. It’s very likely it has layers underneath the plaster so you really have to demolish that shit.

Thirdly you can buy like really concentrated vinegar, like the one that you can’t smell without coughing like resident evil patient zero. Take a roller, dip it into vinegar and roll over the mold. Do it at least 2-3 times, with 1 week interval between each time. Ventilate the room, or leave it for a couple of days until vinegar smell will weaken, it will smell like witch’s fingernails mixed with lemons.

Mold loves stuffy rooms. Be sure that your room is properly ventilated. But most importantly- find the source of moisture. When this crap crawled on my walls, I had to basically scrub them until I reached the cement layer. And I found two fricking cracks two meters high on my walls. From ceiling to the floor, all moist and mushy. I had to then use vinegar, some anti-mycotic substances and actual hydroisolation which masters use in bathrooms.

1.5 years later and still no mold for me. Good luck and seek advice from masters on this matter, they might help you better than me.