Presence of moving thin blue frame moving on screen

Please read the following user tip and restate your question/problem in detail: 

     Writing an Effective Apple Support Communities Question - Apple Community

What model and year is your iMac?

What system are you running?

Have you updated the system recently? If so when?

Include the information on the About this Mac pane except for the Serial No. of the iMac or a screenshot with the S/N redacted.

What year model iMac and what macOS version is it running..?

see > Identify your iMac model - Apple Support

First thing that comes to mind, is a change that you made in the Accessibility settings.