This doesn't make any sense. Muller was name special counsel in May, 2017. Mueller was replaced by Comey as head of the FBI in 2013. Mueller was working as a private attorney at the time of the meeting. The meeting took place in 2016, well after Mueller left the FBI, and well before he was named special counsel.
Similar article title shows the connection.
EXCLUSIVE: Russian At Donald Trump Jr. Meeting Worked With FBI While Special Counsel Robert Mueller Was FBI Director
http://truepundit.com/russian-at-donald-trump-jr-meeting-worked-with-fbi-while-special-counsel-robert-mueller-was-fbi-director/