Cloud Photos is for syncing between your devices and replacing your photos if you lose your device. It retains a full copy of your photos in iCloud (accessible on iCloud.com as well). iCloud Backup is a one-way backup of your current photo library on your device. It can only be restored by restoring your entire iCloud Backup. You can safely turn off Photos from your backup - unless you prefer to manually manage your photos, I'd recommend doing this instead of turning off iCloud Photos. If iCloud Photo Library is on, there will be a note saying "Photo Library is backed up separately as part of iCloud Photos" - if you don't see this message, double check that your device is actually syncing with iCloud Photos. You can do this in the Photos Setting or in iCloud Settings > Photos.