Watermark on iMac screen

I've got what seems to be a watermark on the right side of my iMac screen. It seems to be growing/spreading.

I tried taking a screen grab to include in this post, but it doesn't show up on a screen grab. I took this pic with my phone, which shows the problem.

The machine is several years old and I don't think it's covered by AppleCare anymore. Any tips on what this could be and how I could try to fix it?

Thanks in advance for your thoughts and suggestions.