How to change deck rim joist default material

[Jambruins]

For some reason my deck rim joists have a graphite siding material applied to them.  How do I change this? I am looking in the default setting but can't find it. 

[robdyck]

Just now, Jambruins said:

For some reason my deck rim joists have a graphite siding material applied to them.  How do I change this? I am looking in the default setting but can't find it. 

It comes from the exterior material of the railing wall above it.