That is nothing, just a foreplay of things to come.
the nut is rusted and fused together with the screw and just turns all together and can't remove.
This is an indication that the mounting ring is also rusted.
Cut the screw off, and remove the toilet.
You might get some cosmetic scratches, those you can fix with porcelain patch.
Then be shocked how rusty the toilet mount ring is that you have to unscrew -replace.
But not before you might have to replace the rotten floor.
The seal around your toilet looks weak or non existent.
This gap should be filled with caulk.
There is also indication of mold.
1- Wax ring (make sure you get the right hight)
2- Porcelain patch
3- Toilet mount ring with screws (S/S preferred)
4- Caulk -silicone
It should be against the law to sell iron screws in wet areas.