I've noticed a couple of things while watching MCZ performances; call them individual quirks from some of the members.
Momoka, I think, likes to twirl around while dancing. During the 2016 Toujinsai performance (Day 1 maybe?) of The Golden History, at the very end, she's the only one that twirls around in a circle before singing the final line. Also, I noticed the same thing during the Guuchoki Party Coconuts instructional video. During the chorus, the girls all wave their hands out front, and Momoka is the only one that runs around in a circle.
And this one is more obvious, but A-rin likes to make faces and be silly in front of the camera. She does it during performances and during other shows. Sometimes she'll do it for a LONG time, too long. Pretty silly, but cute.
Have you noticed any other performance quirks from specific members?
Reni likes to do heart signs to the fans when theyre on the carts (did that 7 times during WE ARE BORN at Kuri). Her costumes also tend to fall apart sometimes... Especially hair pieces....
Maybe not so much a quirk, but there is Shiori's infamous jugular vein. It really pops out when she sings, and once you notice it it's impossible to unsee it.