Should I mention the mutual acquaintance who recommended me to a professor in an email?
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.