Hiding/password protecting photos

As correctly stated above, no.

If the photo has been moved from Camera app and your camera roll into Photos app, you can hide it. To do that, select the ... button and select Hide. Moving your photos from your camera roll into the Photos app involves using iCloud for your photos, or using a Mac or PC to store and manage your photos, and then synchronizing your iPhone with your Mac or PC Photos library.

Which means...

Set up and use iCloud Photos - Apple Support

or...

Transfer photos and videos from your iPhone, iPad, or iPod touch - Apple Support

Sync photos between your Mac and iPhone or iPad - Apple Support

In addition to setting a passcode or a more secure password as was correctly recommended above, log some feedback with the folks that work for Apple, so that there might be an easier way to do this:

Product Feedback - Apple

No, you cannot password protect photos. However, you can password protect your device so that no one can access any of your photos.

Set a passcode on iPad - Apple Support