Rot in bathroom floor (Ed.)

[Building_novice]

Bit of a gross one but we are renovating a bathroom, the house is old, looks like lots of water had been causing havoc spilling round the edges of the bath for some time, there was viable mildew spots between tile grouting but we have since ripped out bath, chipped off all old tiles and found this what looks like a giant mould tree growing on the wall… between the tiles and wall!?! Albeit looked very dry/brittle. The bathroom has been airing last few days, damp and weak joist repaired and plumber doesn’t seem concerned about this mutated mould “plant” situation- says they’ve stopped the leak, aired a few days and happy to put new tiles on (post plasterboard and some rendering work…) anyone come across such a thing before and can validate if we will be okay to tile over this or do I need some sort of mould exterminator ghost buster style!?

 

[blup]

Identify what it is and remove kill it off before tiling surely. Post pics for advice