I want to be careful about making a judgment about whether your step-dad is being “normal” on the basis of only 5 sentences. What I can tell you is this: Many fathers and stepfathers find it difficult to deal with their feelings when their daughters reach the age when they are becoming physically attractive young women. They are living in the same house with a young woman who reminds them of what it was life to be young themselves and who looks like someone they might have wanted to date when they were teens. Their feelings of attraction are normal. But here’s the important thing: Responsible fathers, stepdads, grandfathers and other male relatives understand to their core that their job is to protect the young women in their families. This means keeping clear boundaries themselves, and it means making sure that others do the same. They would never act on their feelings and often feel terrible that they even have them. They understand that maintaining a loving and safe relationship with you is part of what prepares you to eventually find someone to love who loves and respects you.
Your step-dad may think he is only complimenting you. But I completely understand why it weirds you out when he talks about wanting you to be his “secret girlfriend.” The fact that he says it should be secret underlines that on some level he knows he is being inappropriate. It is inappropriate. Instead of making you more comfortable, he is doing the opposite. You justifiably want to keep your distance.
It is absolutely okay for you to tell him that he is making you uncomfortable and that you want to be treated as a daughter, not as a girlfriend. But the responsibility for maintaining clear boundaries shouldn’t rest on you. If he keeps it up, tell your mother what he is saying to you and ask her to have a talk with him about the importance of being a safe adult male in your life. I hope he shows himself to be a quality guy by apologizing to you and your mother and never, ever making such comments again.
I wish you well.