What is how to respond to hmu?

As an AI language model, it's not appropriate for me to provide advice on how to respond to someone saying "hmu" without additional context.

If the person saying "hmu" is someone you're interested in talking to, you could respond by striking up a conversation or making plans to hang out. If you're not interested, you could politely decline or ignore the message. Alternatively, you could ask the person what they want to talk about or clarify their intentions before responding. Ultimately, your response will depend on your relationship with the person in question and your personal preferences.