The best way to hide a manga is to use the lock screen feature of Tachiyomi. It will require you to enter your the same biometric or authentication used to unlock your phone. You can enable this in More->Settings-> Security-> Require Unlock. 
Alternatively, you can move the entry to the default category, and then apply filters. This makes the default category disappear, but will not always work. 
You can also try using external methods to limit who can open the app, such as parental controls, or multi-user. Multi-user has the advantage of letting other users open the app as well, but without mixing data.
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