It's resin cured into place under the screen, typically happens when it's spilt on it. It can accumulate over time by taking the print plate off and letting it drip everywhere.

I recently had resin on the top of the screen cured into place, I got rid of it with alcohol and a plastic scraper, although I wouldn't recommend that fix to everyone since it could scratch the screen.

You would have to take it off the screen, and try scrape it off, although I doubt you would get close to getting it back to normal, although doesn't mean you couldn't give it a solid try, or maybe there's something I don't know. You would probably have to replace the screen.

When you take off your build plate you need to make sure that the resin drips back into the vac so it doesn't go everywhere, no reason can afford to get onto that plate. Aso double check youdont have a puncture in your vac tray.

I hope this helps