I've never used a compliment like that, so I am definitely not sure but I will give my opinion. He probably means that your true beauty comes at certain times, and he finds it endearing or special. Alternatively, he just heard that somewhere and though it sounded good so he wanted to use it. When I compliment a girl I like to be specific, and tell them what exactly it is that I like about them. "I love the way your nose moves when you talk," or "the way you jump up and down in excitement" probably less ambiguous.