Yes, the sentence "Say that thing you have in mind" is grammatically correct. It means that someone is being asked to express or speak about an idea, thought, or opinion they are thinking about but have not yet shared.
Here are a few possible rephrasings:
1. "Share what you’re thinking."
2. "Go ahead and say what you're thinking."
3. "Please express the thought you have in mind."
4. "Tell me what you're thinking about."
5. "Speak your mind


