I would define mental safety is something we feel confidence in what we do.
For an example, even in this 6S question forum, I should feel that no matter my definition will be agreed by others or not but with a confidence, I should write this answer.
Also I would say Mental safety is little more important than Physical safety because it might be the cause for a physical hazardous if we mentally not calm and focused on what we do physically.
In such a way it creates impact to the quality of work we do. So have belief and focus on what we do to feel mentally safe.