Hi, this is about the same type of foam as available in
building-supply stores for insulation/filling cracks. You spray it in
and it expands and sets up; that is the way it was put there
originally.It was sound deadening, and to help keep engine noise from
entering the passenger compartment through the hollow space in the
rear body pillars.
I dug mine out; you can probably melt it out with a strong solvent
like lacquer thinner. I would put it back after repairs are made;
just paint the area first if you fear rust.