Douma made a potion by himself out of fun. He wanted to make Lord Muzan a bit of proud, but still, it miserably failed. Douma really thought he made a healing potion which was wrong. When Douma gave Muzan to drink it, The only result to appear was a Neko Muzan. Muzan was now fuming furious and almost kicked the hell outa Douma

From that, Muzan became pretty embarassed, as he was now alone in his own room. Also by unexpecting stuff, Kokushibo also knows about that??