I have a set of fiberglass steps that has metal wire embedded into it bur covered in fiberglass. We are now getting in the 2nd and 3rd year some small rust spots that don't want to come off ; least with a normal rag or with your fingernail .

My supplier says it is due to something put into the pool, but we've not added anything other than salt water magic mix on a monthly basis.

Thoughts on what caused it and how to clean it?