When I see mold starting in beehives, I try to find out why and correct the issue. Some reasons is because the hive is situated at an angle that lets water run "in" instead of out so it doesn't get trapped. Or a super or migratory top cover was vented to allow water in.
Regardless, if its inside the hive, I try to replace the part with a newer part (if you have any) and then I'll bleach/rinse the moldy hive and let it air out for a long time before reusing.
If this is a LIVE beehive, do not use bleach or any other cleaner. The problem is moisture buildup. The cure for it is to give them a vent for that hot moist air at the top of the hive. If you use an inner cover, notch one side and move top cover till it is open, OR use a couple popsicle stics between the inner cover and top cover till it opens it up which will release the moist air. Your bees will clean out the mold after it starts to dry out as long as the population of the hive is good. They are excelent house keepers..
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