What is that black stuff under my carpet?

So, my husband and I started living in our new apartment in Toronto. Last day, I pulled back the carpet in order to study the situation of the floor as we were planning to renovate the house and seek help from concrete flooring solutions. There was black mould all over the surface and I am confused on what this really is.
It is a mud-like substance, which is somewhat hard and can be scraped off and crumbles to dust. This stuff also is spread across the entire room.
Can anyone tell me what this crap is? I am really hoping this is some type of old glue and not a mould problem.

I think they are mold spores. Inhaling or touching them may cause allergic reactions to sensitive individuals.