"open mind" behaves as a noun. "open minded" behaves as an adjective.

You can use "open-minded" to describe a conversation or someone: "I met my new professor yesterday. She seems very open-minded, so I'm looking forward to taking classes with her!" You can say "open mind" (no hyphen) like this: "I need you tell you something, and I know you're probably going to say 'no', but before you answer, please have an open mind about what I have to tell you." In this example, the speaker knows that the person they are speaking to already has an opinion or feelings about something but the speaker would like for the listener to be willing to consider and listen to what the speaker has to say. Hope this helps!

When you use “open mind”, you’re expressing your (or someone else’s) willingness to accept other opinions, ideas, or beliefs that aren’t your own. OR if you’re suggesting that someone else has this approach. Examples: 1. He walked into the meeting with an open mind. 2. I’m going to keep an open mind when I start dating again. 3. Please try to keep an open mind when you try new things. Open-minded is used when describing yourself or someone else who already has an open mind because it is already their nature. Examples: 1. I’m a very open-minded person because I love to learn about other cultures and beliefs. 2. I feel very comfortable telling him how I feel because he’s such an open-minded person. 🌈 Nicky

I'll add my explanation. "Open mind": "Open" is an adjective describing the noun "Mind". And of course, it means that one are willing to consider many things. One doesn't have a fixed judgement or preconceived ideas. Examples: She is keeping an open mind. You have an open mind. I will maintain an open mind until I hear everyone's story. "Open-minded" is a compound adjective. Examples: She is open-minded about her future. I try to stay open-minded. I get along well with open-minded people. There is one more to add to your collection: Open-mindedness is a compound noun referring to the quality of having an open mind (or being open-minded). Examples: I admire your open-mindedness. Open-mindedness is an important quality for a judge. My open-mindedness sometimes gets in the way of making decisions. I hope that helps a little.