Q: Slideshow creating grey lines in photos

Post a screenshot of what you're seeing? 

What version of Photos are you using? 

What theme? In all images or just some?  If just some anything different about them, i.e. where they are from, format, etc.?

Post a screenshot of what you're seeing.

Have you tried repairing the library according to the instructions in this Apple document: Repair Your Library?

Warning: If you use iCloud Photo Library with Photos, this action will cause Photos to upload all your pictures again (from Fix missing or incorrect thumbnails in Photos - Apple Support).

Also export the original versions of some of those images to the Desktop and view them with Preview.  If they display the gray lines then the files are damaged and can't be fixed.