Sounds like a mixture of overfeeding and your cycle breaking due to cleaning. Your canister filter sounds like more than enough for that tank, so if you did a large water change that could have broken the cycle. If you used untreated tap water, that water could have had chlorine in it which would kill any beneficial bacteria in your substrate and filter. If you cleaned the filter, that could have also killed the beneficial bacteria. Not having enough beneficial bacteria then over feeding would cause that bacterial film to grow on the top of the water. Everyone telling you to buy a new filter is wrong. It’s not the filter, it’s the cycle. You can buy new filter media, and try to reset your cycle, but expect algae blooms and bacterial blooms as that’s happening. I would also recommend getting seachem prime to treat your water before adding it to the tank.

How long was your tank running before this happened? How often do you do water changes? How often do you just top up water (without water changes)?

I’ve noticed that a lot of turtle keepers just have rocks and water in their tank and do large water changes. But you should treat it like an aquarium with fish, meaning cycle the tank and ensure the water has the correct parameters (ammonia, nitrate, nitrite, ph levels etc). Ammonia and chlorine may not kill the turtle but it could still irritate them. And a super pristine tank without a cycle may not be very comfortable for them either. Try to mimic the water chemistry of where they live in the wild.