The fish probably would not touch it. Unless you have large predator fish, accidental ingestion would be unlikely
In terms of chemical reaction when the screw corrodes, here are some scenarios:
If it is a copper screw, that
could be bad depending on the size of the screw relative to the tank size and filtration system.
If it is iron based, the rust would mean a small amount of free GFO?
It might not even corrode at all depending on the material.