As above, store with skates out of bag, insoles out of skates, skates opened up in dry place.
If you think the insoles are washable ie not leather, I would soak them in cold water mixed with baby equipment steriliser such as Milton, then rinse well and hang to dry. (Remove from water when black spots are gone.) Dry completely. It takes black mould spots off horse water buckets in c 2-4 hours.