Since Kaki is a top idol star, he put on a disguise by wearing a wig and mustache. According to the article I read, Young Saeng was embarrassed doing the scene but later on had fun with the wig and mustache. If you still remember a few weeks back he even showed us a selca of him in a wig which we all thought from the MV he was doing.
If you have KBS2 you may catch 'Sent from Heaven' early evening.
Let me share here few screencaps from previous episode that I got from SS601.com. Thanks to 그아이taran님영상캡쳐.