If this tab opened by itself while you were setting up Clerk.AI, that was the app checking it can drive your browser — and it can, because you are reading the proof.
This is the same mechanism Clerk uses during a real period close: it opens your bank and card portals in a browser on your own Mac, the same way you would, and stops for you at every real login. Opening it was counted as a visit, without cookies, the way any page on this site is (the privacy policy says what that records); nothing else about you, and nothing from your computer, was sent to it. You can close this tab — the app will carry on by itself.