Could be just dust & cobwebs. My unused chimney was very dusty & had cobwebs & old leaves when I lask looked. Never light a fire do shoved a chimney balloon up there.

Your home is an old home, therefore, once upon a time the fire place was probably used. The dark colour is probably inbuilt soot, from when the wood fires were burning many years ago, and dirt from not being used. I doubt whether it is mould.

Sorry I meant the greenish/grey part on the top middle

Looks like chipped rock to me.

Hard to tell from photo. Is there moisture?

Can’t tell what you mean from the photo.

Belinda, are these mould issues all the one property? Get a mould remediation company in to advise. Mould is a living organism and embeds itself in porous items including soil. If the area under the house smells musty that is mould. Indicating sub floor ventilation and or your drainage is not keeping the area dry. This will contribute to damp in the house. As will single pane glass in bedrooms on south sides of house.

I’ve had mould issues before, and the advice about identifying the root cause is crucial. Simply cleaning the surface never solved the problem for me until ventilation was addressed. The step-by-step process described here matches what professionals actually do. Using proper safety gear is something many homeowners underestimate. This guide clearly explains why prevention is just as important as removal.