If you have read through all your messages in the messages tab and still have a red notification, it may be because you have not responded in the chat yet. The notification will remain to help remind you to respond to the person.
We encourage you to not linger in responding if you have read a message. As the best way to get to know someone is by chatting and actually getting to know them.
If you are not able to respond in a timely manner but have already read the message, you can simply let the person know you have read it but may need time to respond.
If you do not wish to continue to get to know the person, simply kindly thank the person for their time and honestly let the person know, then end the chat once the person has read the message.