Mystery carpet stains - help!

[CarpetTroubles]

We had a spillage a year or two ago which was cleaned up with soap and water. However, this seemed to make it worse and we were left with a grey water stain that I couldn’t shift with carpet cleaner. This grey patch seems to be expanding despite us not touching it. It’s now about 10x bigger than it was. Does anyone know why this is happening and how I can remove it? It’s really ugly and it’s in the middle of the room so hard to ignore.

Photo attached.

This is likely to be a fungus if expanding. What's the carpet layed on? I would check this first and floorboards may need to be taken up and removed. Alternatively concrete may need professional dehumidifier

Under the carpet is underlay and then concrete. It’s definitely not damp. Our flat is very warm and dry.