Got a leak in some flats, from the shower waste in upstairs flat. This photo was taken in hot water cylinder cupboard in downstairs flat when leak is running to.
What's the brown stuff? Some sort of fungus? It was soft, damp and easily removed.
After repairing the leak and drying thoroughly all affected areas you need to sanitise where the water has tracked with a bleach solution.
If you don't sterilise it the fungus spores will continue to grow at the slightest sign of damp.
What I had once many decades ago was Chicken o' the Wood. It was a scary bright yellow fungus, but fried with butter it was absolutely delicious, very dense and meaty in texture.
No - you need to remove the cause, if it is 'wood ear' then it is very likely growing out of very wet wood, if the dampness can not be stopped (such as in a cellar) then either the wood needs to be removed or replaced with something more suitable.