Hi wssviper,
There is some good information about cleaning a mattress and spot cleaning any stains here and here and here.
If it’s a water stain then the suggestions that are linked in post #2 here should be helpful.
If the cover is washable then you could also wash it according to the manufacturers instructions (which I would follow very carefully). If you can’t wash it then removing it would probably allow it to dry more quickly and completely.
Phoenix