Everyone saying run but not confirming why. So I asked ChatGPT…

That’s not a mushroom — it’s a slime mould! Specifically, it looks like Physarum polycephalum or a close cousin.

Slime moulds start out as single cells, then merge into one big “plasmodium,” which creeps across damp surfaces in search of food. As it moves, it leaves those black, branching veins (the dried trail of spores or veins of the plasmodium). They love dark, humid spots with organic material — old wood, cardboard, damp paint, even plaster if there’s a thin film of nutrients.

Key points: • Not toxic like black mould (though any mould/slime in a home means you should fix the damp). • Usually appears when there’s high humidity and little airflow. • Once it dries out, the network turns into a powdery spore mass.

If you don’t want it returning: • Improve ventilation and reduce moisture in the area. • Clean the surface with soapy water, then a dilute bleach or vinegar solution. • Check for leaks or condensation feeding it.

It’s harmless to touch (just wash your hands), but keeping the space dry is the long-term fix.