Green lines on my iMac screen

hello all, I've been wondering if you can help

I've started seeing these subtle green lines - especially if I close or minimize window of Safari or Photoshop.

I have no idea why is this happening and reboot does not help - lines are only more subtle.

It looks like lines are in a shape of closed / minimazed window.

It is late 2013 model and I've never had issue before, only HD has been replaced last year May.

To me it does look like maybe the cable is not working / has been damaged as service opened it ?

Thanks in advance.