Thank you, Clicio!
I think it’s just wet, or maybe some algae. The past few days we’ve got really rainy, humid weather. I placed in half shade outside, covered from rain.
Here a closer img from that black thingy. It’s hard,like the bark around, not looks decaying.
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If it’s a problem, just tell me. I’ve got it from an official bonsai nursery so if something off, I’m sure I can arrange with them.