We always run the fantastic fan when we shower, it's in the bathroom and takes out all that humidity pronto!
You might consider a dehumidifyer.
As to removal, bleach is the answer.
Bear in mind that the under side of the mattress will get very cold as it hangs "in space" so depending on insulation you may be getting condensation. Consider a thin sheet of foam insulation under the mattress, as wide as the slide out or better, to act as additional insulation and a vapor barrier of sort.
We noticed the leading edge of the mattress gets wet or damp in 40 degree or worse weather. We just pulled it away from the front wall by an inch to let air circulate, no slide in our
bedroom. Thank God we never got mold.
Good Luck and let's see what other solutions are offered.