Queen excluder should have nothing to do with,..too much room for a small winter population is most likely the culprit.
Dead bees on the bottom with poor ventilation can add to the problem.
Take honey super off before winter is important.
Do early hive inspection in spring, clean, move combs, some outer honey frame move in a little.
Perhaps they only need one box now,...that's what they should have and no more.
Mold doesn't happen in a box full of bees.
Perhaps salvage some mold frames, scrape off mold and see if they take it, set the box aside.
Sometimes I feed honey combs on top of the hive cover in another box. Hive cover would have a hole in the middle so they can come up and take as needed in spring and fall,.taking it down to fill empties.
They don't consider honey above their home, [outside/above cover] to be their home, so you can call they would rob the honey.