I'm sure plenty of jokers will chime in......
but, if you ever experience this, there is a way to eliminate it.
Upon super chlorinating or super-brominating, using a clean rag or sponge, take the water from the spa and wipe the topsides of the spa above the water line. splash and wipe the underside of the cover as well.
this will kill any mold, mildew or bacteria. little black dots on underside of cover(the start of mildew).
works like a charm!