You don't say what device you are using, or what OS you are running, so I have to make a guess:
For a Mac, the password used for Hidden Photos is your computer password-- if someone has that, then they have access to your entire computer and to all your files, and an extra password would not help. Photos is not intended to keep that sort of security-- you shouldn't keep copies of private documents like deeds, drivers' licenses, password lists, lock combinations, etc, in Photos, hidden or not. It sounds like you need to change your computer password.
If you have pictures or other files on your Mac that you would like to keep secure beyond your computer's password, then you can store them outside of Photos in an encrypted folder. You can create a password protected encrypted folder in Disk Utility with File>New Image>From Folder.