Any way to get rid of this mold?

Found this branch with moss growing on it and started a terrarium. It's the only thing in the jar right now and its growing mold now. Was previously sealed, I'm letting it air out a bit now. Is that a bad idea?

Found this branch with moss growing on it and started a terrarium. It's the only thing in the jar right now and its growing mold now. Was previously sealed, I'm letting it air out a bit now. Is that a bad idea?