... i'd spray Exit Mould (and don't use the cheaper versions) on it and leave for a few hours BUT check on it every so often so that it's not turning any other colour like orange or pinkish tones cause if it is then rinse it off with water asap !!

Make sure there is ventilation when doing so, like leave a window open to the bathroom ... be careful with the Exit Mould to not get on clothes, any fabric as can bleach them ... also if your even the slightest asthmatic when using maybe wear a mask !!

Also be careful with taps as can give a rust effect ... so sorry so many warnings here BUT this stuff is powerful and truly WORKS