Coloured line at top of Mac screen - only when watching video

Hi

i bought a refurbished Mac a few months ago and it has been great. Today I’ve noticed that there’s a coloured line at the top of my screen - this is only visible when I’m watching video like catch up tv or something online. Is this a hardware fault or can it be fixed by a reset?

I don’t know enough to test solutions so I’m hoping someone has dealt with this before!