Don't know what it is, but I had one appear on my old monitor.
I searched, they seemed reasonably common, you can see a thread here with same complaints.
https://forums.tomshardware.com/thr...nitor-more-visible-with-white-screen.1377434/
A lot of people seem to say it's a pressure mark from being poked/prodded. Mine 100% had not been, it just appeared one day. I got an RMA with lg to get rid of mine, it was fine for years after the rma, then another one just popped up. Don't use it anymore, gave it away to my brother, but it still works fine, so doesn't seem to cause any long term issues or get worse.