[Intex inflatable mould / cleaning]

Hi all, got the hot tub out of storage to find what I can only describe as a black mould as well as other stain patches. Does anyone know how to clean/remove these? Even my jet wash doesn’t take it off.  I’ve tried bleach, that does seem to fade it but not remove it

 

 Pretty devastated to be honest, I clean the whole thing down and dried it thoroughly before storing it :(

 

Hopefully someone can help. Kind regards Mark