Gross: 1 of my 3 hives has WORMS taking over

I'm not exactly sure what this is. I was seeing an uptick in small hive beatles and assumed the worms were their larval stage, but then I saw a full grown mealworm looking creature in there today. This was/is a laying worker colony on the decline. I've given up on the hive. Going through and freezing frames for 48 hrs now — only two at a time bc I don't have a deep freezer. 14 more to go. 😥 Can anyone explain what happened and what I can do to prevent this and ensure it doesn't spread to my other hives?