I just tested on my Mac with macOS27.
The + button appeared, after set the view to "All photos" and I started to play the memory movie, then paused it and switched to the grid view. The + button is not really a button, it is the last photo shown, marked with a "+" at the end of the grid of all photos.
When I click it, it suggests some photos to add, but no access to all my photos.
But I found no way to remove photos from the automatically generated memories. The artificial intelligence tends to favor the most silly photos of people, that I would never want to show in a memory.
When I want to modify a memory video, I prefer to create new one instead, by adding all photos in the Memory to a new album and edit this album. This way I can add and remove photos freely, without the artificial intelligence interfering and trying to be helpful. Then I play the album as a memory and save the memory, see:
Watch memories in Photos on Mac - Apple Support
- "Create a memory from an album: Click an album, then click
. Click 
in the toolbar to add the new memory to Memories."
on your Mac.
on a memory to play it, then click
.
. (You may need to scroll down.)
