Inner rocker repair - dumb question

[Sunnyledge]

I am starting to repair a rusted floor section in the passenger footwell. I have a small section of rust in the innerrocker. I've cut that out to repair and have encountered a grey fluffy material in the space between inner/outer rocker. Is this supposed to be there or was it deposited by a rodent?

[D.Martijn]

When I repaired some rust on my touring there wasn't any "fluffy material"..
Do you have any pictures?

[Sunnyledge]

Now that I've remove some more of the material, it's pretty clear that it was not installed by BMW. Not sure how the rodents got in there.