There was no secret meeting. Comey asked to speak with Mueller. The Senate Intelligence Committee was informed of the meeting beforehand and it was reported in the news. Since Mueller is running a criminal investigation it was necessary for him to meet with Comey before his public testimony before the Senate Intelligence Committee. Jason Chaffetz, the Republican chairman of the House Oversight and Government Reform Committee knew about it and agreed with the need for the meeting. There was nothing nefarious or secretive about it.
Mueller met with many witnesses during the investigation including Jared Kushner. That's part of his job, to talk to witnesses about their testimony.
This is another attempt to discredit and smear Mueller and his investigation. Why would that be?