What is this spongy material that the walls on this old house are covered in?

This?

I just bought an old empty house across the street from me, and most of the downstairs walls are covered in this material. It's about 2-3mm thick and underneath is 土壁. It's almost spongy and can be picked off like little bits of foam.

I'm pretty sure it's some type of insulation material, but I've never seen it before. Was wondering if anyone knew what it was.

EDIT: I believe it is called 繊維壁 (seni-kabe) and is a fiber-based material that has the potential to have asbestos in it, depending on when it was put up. I'll be contacting a professional!