If you can clean it off and it's only on the positive tab, I don't see it being a problem. Otherwise if it is on the walls of the Cell, heating and cooling will cause the rust to keep going (galvanic action), eventually leading to a hole. This is the biggest reason to avoid nickel coated steel strips. If the steel starts rusting .....
So surface rust if it is removable, remove it. Just know to keep an eye on that cell.