Keep photos on icloud Photo Library but delete from Mac Hard Drive?

iCloud Photo Library is designed primarily as a syncing method and is not designed to be cloud-only storage. If you have iCloud Photo Library turned on in your Mac and you delete a photo from the Mac it will be deleted from iCloud and from any other device connected to that iCloud Photo Library. You can reduce the amount of storage space required on your Mac by opening Photos and going to Photos Preferences, click iCloud in the bar at the top of the Preferences window and then click Optimize Mac Storage.

The following has information about how to turn off iCloud Photo Library on a device: Get help with your iCloud Photo Library - Apple Support