If it is a mold it will be good to clean it as fast as possible. The mold tends to spread, especially in tissues. Here, either try means for mold remover 3M Marine Mildew Remover, Clorox Disinfectant...
I use Sun Joe 3000. It is really great. I am satisfied. I have used some washers made by other brands and this one is on the top. Another one is Karcher K4. It is also good.