So, what can we learn about the person in red from this clip?
It's a woman. Unless the character is a man with very small hands, it seems this figure is definitely a woman.
She's white. This is self explanatory. The hands were not only white, but pretty pale.
She was wearing bright pink nail polish. This seems worth noting.
So, who could it be? We know Meredith (Amanda Schull) is supposed to be locked up, so it seems like she's ruled out. Earlier in the episode, CeCe (Vanessa Ray) was wearing much paler pink nail polish, which makes us think it's not her (though she could have re-painted her nails).
What other suspicious women are left? Obviously, there's Mona (Janel Parrish), though at this point there's no reason to think she ever wears the red coat. There's also Melissa Hastings (Torrey DeVitto), but we feel like she doesn't normally look that pale, and bright pink doesn't seem her style. While you never know on this show, Paige (Lindsey Shaw) appears to be a good guy.