Seems like it wasn’t actually mold, but JK still sucks although that was great:

Somewhere in the distance they could hear Peeves zooming through the corridors singing a victory song of his own composition: 

We did it, we bashed them, wee Potter's the One,

And Voldy's gone mouldy, so now let's have fun!

'Really gives a feeling for the scope and tragedy of the thing, doesn't it?' said Ron, pushing open a door to let Harry and Hermione through.