How can I fix this hole?

Hey, any help to a total noob is greatly appreciated.

New build, en-suite started getting mold on our ceiling, which we assumed was down to ventilation so we started opening the windows etc.

Recently a small brown wet patch appeared next to the tiles in the shower, which has slowly grown. It was very wet and collapsing on its self when touched. I emptied the wet bit out of the patch and everything else around it is dry.

I have 2 questions, how can this happen in such an isolated spot? And how can I fix this?

Thank you!!