horsehair

Hello, I am quite new to DIY so please forgive my naivety if I have done something silly. In have a 1900s terrace house. The internal party wall has an unused fireplace which I believed had hygroscopic salt build up because there was a constant damp patch in the paint and a Timberwise person...

Im in the process of lifting my floors and removing the slag/burnt rock insulation. There is a layer of whitish plaster which I assume is horsehair between the joists. Is this just to keep the slag in or is there some other reason. Ive also got a sickly sweet smell coming from the floor. I guess...