What does the medical cross symbol over a room mean?
According to this Steam Posting, this means a High Priority room:
Green plus symbol means you made that into a high priority room so it will always be staffed before others.
It looks like this means that these kind of rooms will be staffed first over other rooms. Also according to the post, you can remove this effect by:
You can undo that by clicking on the room, and on the window that appears, you will have "Low" on the very right. If you click on it , your room will be back to normal priority.