How to "unshare" a video in iCloud
It appears that you've added a video to an iCloud Shared Photo Stream. Since you are the one that added it to the stream, you can delete it.


  Can I delete photos, videos and comments from a shared stream?
  
  Yes. If you are the owner of a shared stream, you can delete any photos, videos or comments that you or your contributors have added.


Source: Apple KB article: iCloud Photo Sharing FAQ

To delete a video from a shared stream:


Go to Photos → Shared.


Select the stream that you added the video to, then select the video.
Tap the Trash  icon to delete it.


To delete the entire shared stream:


Go to Photos → Shared.
Select the stream that you wish to delete.
Go to People → Delete Photo Stream.


With regard to your statement about using up iCloud storage…


  Do Shared Photo Streams use my iCloud storage?
  
  No. Photos uploaded to Shared Photo Streams do not count against your iCloud storage.


Source: Apple KB article: iCloud Photo Sharing FAQ