Finally, an issue to which a chemist can contribute!
It is probably zinc leaching from the galvanizing. As the surface ages and
as tiny spots of the steel are exposed, electrolytic action takes place and
the zinc is oxidized in preference to the iron, hence protecting the
steel. The consequence is that the oxidized zinc is water soluble and is
toxic to the grass. Living systems need a trace of zinc but only a trace.
When I lived in Washington State I used to get the "crumbs" of galvanizing
from the bulk nail bins at the hardware store and scatter them near the
peak of the roof. As the zinc leached out it would kill any moss and keep
it from regrowing on the roof.
Now, aren't you glad to asked?