Should I mention the mutual acquaintance who recommended me to a professor in an email? I'm applying for graduate school in the coming fall. I asked a professor - who I never worked with, but who liked my CV - if he could get me in touch with one of his colleagues to see if I can get a research position at his lab. The professor did so, and his colleague was interested in my CV enough to tell my professor to ask me to get in touch with him. 

I didn't want to email this professor until I was done researching his stuff and sending my application. It's been a couple of weeks. I don't know if I spent too much time not sending the email, but I didn't want to make a poor job of that. But the question is, now that I'm writing the email, should I mention the professor who recommended me in the subject line?

My current subject line is:
"Cady Heron - Prospective [major] Graduate Student"

The one I was thinking:
"Referred by Dr. Karen Smith - Cady Heron"
(Frankly, I got this from a Business Insider article.)

Any help, please? I just don't want to sound like I'm shoving nepotism or something in his face.
It's a good idea to remind the prospective advisor of who you are (namely "prospective student that colleague X recommended"), but doing so in the subject line might be overdoing it. I'd suggest writing that reminder in the first line of your email, since most people seem to read emails using applications that will display the first line, even if they don't open the email. Also consider cc'ing your referring professor on the email.