TMUX added one more window which I cannot close

I am new to TMUX and tried to ask Google correctly but didn't find the answer. I've been wording on one of the panes which was separated into two windows. One day, I opened the tmux and found out that the third window suddenly appeared. And I have no idea how to close it. I've tried ctrl+b + right arrow. But the cursor didn't move there. So I assume it's not a regular window

Could you advise, how should I close it?

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Thanks