What is this?

Emiemigro

Moved into a property last month and basically every room is caked in mould due to poor ventilation and the roof being ruined. We knew there was what seemed like a small patch of mould in the living room, Stripped the living room wall and found this mess (pic attached) previous owners had obviously cleverly wallpapered to cover this what looks like hole in the ceiling and the extent of the damage. Any ideas what could be causing it and how to fix it? There seems to be mould growing in every corner in the house and above windows and it’s massively getting me down. No windows are trickle vented (installed 2013) and are very bad with condensation. We have dehumidifiers already. 

Carrot007

You would need to find out what is on the other side and maybe getting though.

Stop that. Then bleach bleach bleach. Foillowed by maybe bathroom type paint layer if the source cannot be found, as a just in case, may just be crappy previous owners.

How old is the property? Mix of plasters? Maybe lead pipes in wall for old school lighting (I had that in one!) may route water?

RelievedSheff

That's just mould and some old plaster that has cracked.

Condensation tends to collect in the corners of rooms like that where the surfaces are at their coldest.

You need to heat and ventilate the property properly and the mould problem will eventually disappear. The dehumidifiers will be helping but you obviously have a lot of moisture in the house for it to be growing so much mould.

Have you made sure that the building is watertight? You won't get to the bottom of the damp and mould if you have water leaking into the building.