I never really exfoliate my back so bought one of the BS cactus back brushes for this. Really liked it and have used it a couple of times.
I always put it on the edge of the bath with its bristles pointing downwards to drain the water away.
Only..... It seems to be retaining water amazingly well, and has now gone all mouldy!! It's as though it wasn't meant to get wet or something 
Would you try weak bleaching it? Buy another? Or is there a decent alternative??