What this soft and damp material below my utility room tiles?

[jacoscar]

I’ve started removing tiles in my utility room and found the adhesive/concrete below is quite wet and soft. It seems to be 1-2cm deep and then the concrete seems harder below that
This seems to be only on the central part where a wall seems to have been.
Here’s a photo and a video

I’ve also observed some dampness, especially near the door.

I intended to prime with SBR, self level and then tile, maybe using a decoupling membrane. Is there anything additional that I need to do now that dampness has been identified?

 

[jacoscar]

Update with some pictures taken after letting it dry overnight. It looks less damp now.
I put some cling film down with a glass on top. No signs of condensation, although maybe I should have taped it down

 

[stuart45]

Looks to me like what's left of the DPC that was under the wall.

 

[ree]

OP,
Looking at the edges of the floor, where it abuts the walls, I dont see any membrane (DPM)?
This implies that there was no DPM under the original floor - & maybe what you are calling "concrete" was indeed a concrete floor?
Perhaps, a screed was laid on top of the original floor: the 10mm - 20mm "quite wet and soft" that the tiles are laid on?

To find the source of the damp: remove all the wet & soft screed to expose the original concrete, & the remains of the wall.
The remains of the wall look suspiciously damp. Could be that the damp near the door is tracking from the remains of the wall?

FWIW: the wall plaster should break contact 50mm above the FFL.

 

[jacoscar]

I checked again today and it looked dry; I even checked with a moisture reader and it was about 0.8-0.9%.

So I went ahead and self levelled