it is real
back up about 1 hour and a half in this video and you will see it (it is not possible to give a time stamp since this is live)
https://www.youtube.com/watch?v=Ga3maNZ0x0w

edit: it is already viral
http://www.cnbc.com/2017/07/12/someone-held-up-a-buy-bitcoin-sign-during-yellens-testimony-to-congress.html

edit 2:
i love that guy's face, it is less than a second and he knows he is being naughty