What happened to this leaf? and what should I do?

[Selmo]

Looks like pseudomonas, a bacterial infection cased by warm, humid, still air. Looks like it has coalesced and is inactive now, but could become active when conditions become favourable agian. Remove the infected leaf soon. The bacteria moves by splashing water, so don’t water from above, set the pot in a bowl of water and let it soak up the water. Increase air flow around plant with a fan, and give it a little more light if possible. Good luck with it, the plant should recover.