Why is my icloud drive data showing up in my startup disk storage level

I use a macbook pro to store and edit my photos and a macmini for everything else. I back up finished photos to icloud drive.

I recently kept getting a message that my start up disc was full. On investigating it would appear that it thinks that the icloud drive data is part of the start up disc and is showing 100gb of itunes data.

how do i stop it from doing this. i thought point of icloud drive was that it relieved storage issues but mine seems to be duplicating?