Don’t panic !
It can be a common issue. The key to preventing mildew is keeping the chemicals properly balanced in the hot tub water.
If mildew does occur, mix warm water and mild dishwashing soap and with a soft brush, scrub the area to remove any mold or mildew.
If the mildew is on the inside of the vinyl cover, apply the same cleaning method but make sure to place the vinyl in the sun to dry. If the inside foam core news attention, clean and dry in the shade. Make sure everything is dry before reassembling your cover.